SOHO Images of the sun taken around the summer solstice
A visible light image taken on the 17th June, showing sunspots
X-ray image showing bright areas where sunspot activity is high taken on the 22nd June
Part of the MPEG movie made by the LASCO instrument for 20th June showing the coronal mass ejections along with the bright planets Jupiter and Venus at the left
All of these images are from the SOHO web site.
The Editors Bit
To me the summer time is a time for doing other things apart from looking
at the stars and planets. For one thing you can‘t see any, even if
its not cloudy or hazy, it never gets dark (well it never ever gets dark
in Bedworth unless there‘s a massive power cut!). So do I hang my
head in shame at not being a dedicated observer? Should I get up
in the early hours to see the latest comet. I did try to see Comet
LINEAR in July through the clouds, but failed. This year the summer,
if you can call it that, has been poor all over northern Europe with the
south getting cooked. Even away on holiday in early June in Rhodes,
I hardly saw any stars through the light pollution and the slight prevailing
heat haze. Even dedicated solar observers have had a tough time trying
to see the Sun this year. I have had a few glances at it and I‘ve
not yet seen a naked eye sunspot but I have seen lots of smaller spots
through projected binocular images. So you see I can‘t be a dedicated
observer or I would be tearing my hair out with the lousy weather we‘ve
had and I‘m not. But I do keep up-to-date with what‘s going on above
with the astronomy magazines and quick looks on the internet at the SOHO
images of the Sun as on this issues cover. So I feel I‘m not missing
out on the sky and all the amazing things being reported from probes at
Mars, satellites looking at our star, from the new telescopes coming on
stream, from all the new techniques and theories on both TV and radio,
in the news papers and magazines. A flood of discoveries from the
fact that more scientists are alive at this moment than have ever lived
before. All of this information is to much for anyone to take in,
so that‘s why people specialize in a subject. You can get to know
quite a lot about a small part of the whole cosmos, but it‘s very difficult
to know a bit about all of it. Even when you think you have a pretty
good grasp of your favourite subject, some bright spark will find out a
new fact which will upset the standard theory and off you go again!
Is there water on Mars? Why is the universe expanding faster?
Are there Earth sized worlds orbiting other stars? Is anyone looking
at us wondering the same thing?
Sunsets on Mercury
by Mike Frost
(If you haven‘t already read the preceding story in the last issue of MIRA, 'Prisoner Cell Block Hg' please do so before reading this article. I wouldn't want to spoil the plot twist for you)
Writing an article to explain the science in a story seems to me like
an admission of defeat; it rather implies that I couldn‘t put the science
across clearly in the first place. However, the orbital dynamics
of the planet Mercury are rather interesting and I think a more detailed
explanation is worthwhile.
The motion of the Sun across the sky of a planet is a combination of
two separate components, the rotation of the planet itself and the motion
of the planet around the Sun. Living on Earth, we tend to forget
the second component, but on Mercury it produces some remarkable complications.
Let‘s start with a few numbers. Mercury is the innermost planet
in the system, it‘s average distance from the Sun is only 36 million miles,
compared with Earth‘s 92 million miles. Because it is so close to
the Sun, the orbital period of Mercury around the sun, Mercury‘s year,
is only 88 Earth days.
On Earth, we are accustomed to the day, the rotational period, being
much less than the year. But that isn‘t true elsewhere in the Solar system,
and it certainly isn‘t true on Mercury. Mercury spins around on its
axis once every fifty-five days; in fact, one rotation of Mercury lasts
precisely two thirds of a Mercurial year. Again, on Earth we don‘t
see any connection at all between the length of the day and the length
of a year. A year is about 365·24 days long and those extra
0·24 days cause all sorts of complications with leap years and so
on. Inhabitants of Mercury are spared leap years, the orbital period
and the rotational period are connected by an exact 2:3 ratio. Why?
Is it co-incidence? Or design? In fact, it‘s tidal friction.
Mercury is one of several bodies in the Solar system where tidal friction
has slowed down the rotational period to a 'resonance', or exact ratio,
with the orbital period.
How does the resonance work? Take a look at Diagram 1, which
illustrates how an observer on a planet in a 2:3 resonance will see the
sun move. The planet in Diagram 1, however, is NOT Mercury, as it
rotates around its Sun in a perfectly circular orbit; we shall see shortly
what happens for Mercury‘s elliptical orbit.
Diagram 1. This illustrates how an observer standing on a planet in a 2:3 resonance will see the sun move across his sky from sunrise, the line shows one revolution of the planet, but it takes another ½ revolution before he reaches sunset!
The 2:3 resonance, I hope you can see, produces a surprising journey.
Consider one planetary orbit starting at the lefthand side of the diagram,
with both the planet‘s rotation and its orbit around the sun progressing
counter-clockwise. I‘ve marked the path of an observer who sees dawn
at the left of the diagram. Note how, throughout the orbit, the observer
remains in daylight. By the time the planet returns to its starting
point, after one orbit of the Sun, the observer is just about to cross
the terminator at sunset. The observer then spends the next orbit
completely in darkness, before seeing dawn again, right at the start of
orbit three. So the actual day, sunrise to sunrise, on this planet,
is the equivalent of two years; three times as long as the rotational period.
You may be surprised that the apparent and the actual rotational period
differ by a factor of three. Why? It‘s because the rotational
period is so slow and the orbital period relatively fast. Does the same
thing happen on Earth? Well, yes, but to nothing like the same extent.
Earth does not rotate once every twenty-four hours. The actual rotational
period is just over twenty-three hours and fifty-six minutes. The
extra four minutes (to be more precise, 1/365th of a year) is because of
the orbital motion of the sun through the sky. We call twenty-three
hours and fifty-six minutes Earth‘s sidereal day (literally, relative to
the stars) and twenty-four hours the synodic day (literally, relative to
the Sun). Mercury‘s sidereal day is fifty-five Earth days long, and
its synodic day one hundred and seventy-six Earth days.
Got that? Look at it another way. In twenty four hours,
Earth spins through just over 361 degrees, slightly over a full circle,
but the Earth‘s orbital motion round the Sun means we have to subtract
one and bit degrees, giving an apparent rotation of exactly 360 degrees.
On Mercury the balance between the two is much more even. In twenty-four
hours on Mercury, the planet has spun through just over six degrees (360
divided by 55), but the Sun has moved, on average, by four degrees (360
divided by 88) because of Mercury‘s orbit. So the net effect is that
the Sun crawls through only two degrees of the sky, giving an apparent
year of around 180 (360 divided by 2) Earth days.
However, there is a crucial complication, which makes things really
interesting. Mercury‘s orbit is not circular. At perihelion,
Mercury is only 29 million miles from the Sun; at aphelion it is 43 million
miles away. Earth‘s orbit isn‘t circular either; like Mercury and
indeed everything else in the Solar system it orbits the Sun in an ellipse,
but one that is much closer to being a circle. We call the 'circularity' of an ellipse its eccentricity. Eccentricity has a precise definition
but all you need to know is that perfect circles have eccentricity of zero,
whereas, as eccentricity approaches one, the orbit becomes flatter and
more 'cigar-shaped'. Earth has eccentricity of 0.017 and Mercury
How does a planet‘s rotational period vary as it proceeds round its
orbit? Not at all. Mercury spins around at the same rate regardless
of how close it is to the Sun (as does the Earth, of course). The
rate at which the Sun moves, however, is a different matter.
The laws of orbital motion were first discovered by Johannes Kepler,
and explained a century later by Isaac Newton. I have already described
Kepler‘s first law, planets move around the sun in ellipses, with the sun
at one focus. Kepler‘s second law describes how planets speed up
and slow down at various points in the orbit. Here is the second
law — the line joining planet and sun sweeps out equal areas in equal times.
What does that mean? Simply that the planet speeds up as it approaches
the Sun, and that the Sun‘s apparent motion through the sky becomes faster.
How much faster? Well, on Mercury the effect is quite extreme,
and in Diagram 2, I‘ve tried to redraw Diagram 1 to illustrate it.
I hope you can see that, while the Sun scurries through the sky in the
further reaches of Mercury‘s orbit, at perihelion the Sun stays in virtually
the same position in the sky. If the Sun is overhead as you are approaching
perihelion, it will stay virtually overhead for several days either side
Note that I have cheated a bit on the diagram. Mercury is a very
small planet, only 3000 miles in diameter, so the size depicted for Mercury
is not at all to scale compared with the size of the orbit. To work
out exactly what happens to the Sun‘s apparent motion needs some careful
calculations, which I have made for you.
My calculations make a couple of further assumptions. First of
all, I assume that Mercury has no axial tilt. This is a pretty good
assumption, as Mercury‘s axis is only 0·1° degrees from vertical
to its plane of orbit, compared with Earth‘s 23·0° degrees.
My second assumption, not so good, is that the Sun is a point source of
light. From Earth this is good for most practical purposes; the Sun is
only half a degree across, but of course Mercury is much closer to the
Sun and the angular size increases as Mercury approaches perihelion.
Nonetheless I think my conclusions hold.
Diagram 2. You can see that, while the Sun scurries through the sky in the further reaches of Mercury's orbit, at perihelion the Sun stays in virtually the same position in the sky. If the Sun is overhead as you are approaching perihelion, it will stay virtually overhead for several days either side of perihelion.
Here are the numbers for selected periods in Mercury‘s 88-day orbit.
At the start of day 1 it lies at aphelion, furthest from the Sun.
In days 1-44 it is approaching closer and closer to the Sun, until on day
44 it reaches perihelion. Days 45-88 are similar to 1-44, in the opposite
order; I‘ve included days 45 and 46 to show this.
How do we interpret the numbers? Well, note that the numbers
the second column are constant; that indicates that the planet spins around
at a constant speed. But the numbers in the third column increase
as Mercury approaches perihelion, because Mercury is speeding up.
The fourth column is the difference of the second and third. You‘ll
see that, at the start of the orbit, the contribution from Mercury‘s spin
is dominant, and the Sun moves relatively rapidly through the sky.
However, as Mercury approaches the Sun, the motion of the Sun speeds up,
and the two terms become closer in size. By day 38, the two terms
virtually cancel out; the Sun only appears to move 0·23° degrees
through the sky in 24 hours. By day 40 the Sun is essentially motionless
in the sky.
But for the remaining 4 days to perihelion, and the four days after
it, the orbital term wins out. The Sun is moving through the sky
in the wrong direction. How will this appear in the sky? Well,
to be honest, from most of Mercury it will be difficult to notice, as the
speed across the sky is so slow. But suppose for a moment, that it‘s
the end of day 37, you have just landed on Mercury at a point where the
Sun has just set. The Sun continues to drop slightly further below the
horizon. By the end of day 40, the Sun is a third of a degree below
the horizon. But then it changes direction, starts rising again,
and some time on day 42, unsets itself. Not until day 48 does the
Sun begin to drop back towards the horizon; then on day 50 the Sun sets
for the second and final time during the mercurial 'day'.
You‘ll see that the Sun doesn‘t drop below the horizon very far, only
a fraction of a degree. But don‘t forget that Mercury doesn‘t possess an
atmosphere; so sunsets and sunrises will be abrupt events. In fact,
the view could be staggeringly beautiful. By locating yourself at
the right point on Mercury‘s surface, you can position the Sun, at standstill,
just below the horizon for days and days, creating yourself a long-lasting
solar eclipse. Moreover, with the Sun four times as close as it is
on Earth, all the really amazing sights we glimpse for just seconds at
eclipse time, the corona, chromosphere, solar flares and so on, will
be four times bigger, sixteen times brighter and visible for days on end!
Far from being the location of a prison, I predict the double-sunset zone
of Mercury will become home to a solar observatory and quite possibly one
of the hottest tourist destinations for the next millennium.
Note that most of Mercury doesn‘t get to see a double sunset, because
of the resonance of day and year. You will only see a double sunset,
if the first sunset occurs between days 36 and 40 after aphelion, or between
124 and 128 days after aphelion. If you are lucky enough to see a
double sunset, then stay exactly where you are on Mercury, and 88 Earth
days later you will see the opposite effect, a double sunrise, and 88 days
later you‘ll be back where you started with a double sunset. So only
two small bands of longitude (about 1·5° degrees longitude across),
on opposite sides of Mercury, will ever see a double sunset. At other
longitudes, the Sun‘s switchback behaviour occurs at different times during
the day. For example, Mercury‘s ?prime meridian‘ or zero-longitude
meridian has been allocated, by the International Astronomical Union, to
the sub-solar point at perihelion. This is potentially a very confusing
location, as it has up to three middays in quick succession, followed 88
days later by three midnights!
Aside from seeing multiple middays, the longitude bands at which the
Sun is overhead at perihelion (two bands of course, on opposite sides of
Mercury) are the location of Mercury‘s tidal bulge. Mercury is a
solid body, of course, so there is no opportunity for liquid tides like
we have on Earth. However, the tidal forces, easily at their strongest
when Mercury is at perihelion, may have forced Mercury into an ellipsoidal,
or Rugby-ball, shape, with the pointy bit facing the Sun at perihelion.
We see that the 2:3 resonance naturally presents one end of the Rugby ball,
then the other, each time Mercury approaches closest to the Sun.
This is why the configuration is stable; any further slowing down of Mercury
would generate a torque on the tidal bulge, which would speed the planet
It‘s natural to ask whether or not any other planets in the Solar system
feature a double sunset. Remember the two requirements, a long rotational
period, comparable with the length of the year, and an eccentric orbit,
so that the angular speed of the Sun can vary. Venus, next planet
out from the Sun, meets the first requirement. The sidereal day is
of 243 days is actually longer than the Venusian year of 225 days.
However, Venus has the least eccentric orbit in the Solar system, even
more circular than Earth‘s. Also, Venus rotates in a retrograde direction,
so that the Sun‘s apparent motion due to Venus‘s rotation is in the same
direction as the motion due to Venus‘s orbit. The angular velocity
of the Sun across the sky of Venus varies only between 3·0°
and 3·1° degrees per Earth day, giving 117 Earth days to one
synodic Venusian day. So there are no double sunsets on Venus; but
there is an unusual feature, the Sun rises in the west and sets in the
east. Except that you would have to wait 58 days between sunrise
and sunset and you wouldn‘t see anything anyway, as the sky on Venus is
permanently covered by clouds of sulphuric acid. Venus is not a likely
Earth and Mars rotate much more quickly (about 24 hours in each case)
and the gas giants quicker still. Pluto is far too far away to worry
about double sunsets. The only other candidates for double sunsets
in the solar system would appear to be sun-grazing asteroids such as Icarus.
However, the few asteroids which have had accurate periods measured have
all turned out to be rotating in hours rather than in days. So Mercury
may well be the only object in the Solar system that sees the Sun change
direction in its sky.
Finally, it is worth asking if any satellites in the Solar system see
their parent planets change direction in the sky. There is one obvious
candidate; our own Moon, which meets the two criteria admirably.
The Moon‘s rotational and orbital periods are in a 1:1 tidal resonance,
which in plain language means that the Moon always shows the same face
to us, daytime 'relative to Earth' on the Moon lasts for ever. However,
the Moon is also in an eccentric orbit, and the Earth‘s angular velocity
through the lunar sky varies between plus and minus 1.5° degrees per
day. The net effect of this is that parts of the Moon, on the edge
of the hemisphere facing us, see the Earth set and then unset itself during
a 28 day orbit round the Earth. From our point of view, on the Earth‘s
surface, the Moon‘s face appears to wobble to-and-fro during a 28 day lunar
cycle. We call this wobble libration and it enables us to observe
more than 50% of the Moon’s surface.
So, I don‘t suppose that Clarissa would be too impressed, but she and
Clive were witness to one of the solar system‘s more impressive (and lesser
known) phenomena; the standstill of the Sun on the horizon of Mercury,
followed by its unsetting and then a second sunset.
Tying the Knot
by Mike Frost
As usual, I base my story somewhere in the vicinity of known science.
Elsewhere, the Gordian Knot was tied around the yoke of a wagon in the
palace of King Gordius of Phyrigia, where it was said that whoever could
undo it would be a great leader. The knot was very tangled and convoluted,
but Alexander the Great sliced through it with a single sweep of his sword.
Ever since, Cutting the Gordian Knot has meant solving a difficult problem
by some unexpected means.
”Dearly beloved, we are gathered here together . . ."
It was a beautiful day for a wedding. This one was special; it
was the marriage of my old friend, Clive, of the Interplanetary Dangerous
Sports Club, and his attractive but rather domineering fiancee, Clarissa.
And I was the best man. I stood at Clive‘s side by the altar, fingering
the box containing the ring. The groom looked very nervous.
Although he‘d spent weeks being measured for his morning suit, it didn‘t
quite seem to fit. Clarissa‘s mother eyed her future son-in-law suspiciously.
Clarissa‘s father, doubtless relieved to be offloading his demanding daughter,
still held her arm. Clarissa gave him a quick smile. ”Not long now
daddy." She looked stunning, a vision in silk.
The bridesmaids looked rather tasty too.
I was so busy admiring them that I didn‘t really pay attention to what
the vicar was saying. Until it got to the bit where he said; "If
any man has any just impediment to this marriage, let him speak now, or
forever hold his peace.”
There was silence in the nave of the church, although you could hear
birds singing in the churchyard. Sunlight streamed through the stained
glass windows, illuminating the rather wild, unkempt, bearded chap who
had just marched in through the church doors.
”I object!!!!” He shouted, in a strangled east-European accent.
There was a sharp intake of breath from the congregation. All
the party at the altar turned round to see who was talking. Clarissa
gave a low moan. Her mother shot the man a look I‘d last seen in
Walking with Dinosaurs.
”I object!!!!” The man cried again. ”Clive is already married!
To my daughter!”
All hell broke loose. Clarissa screamed and threw herself into
her father‘s arms. Her mother gave a small sigh and collapsed to
Clive himself looked puzzled. Then a flash of recognition crossed
”Oh, you must be Olga‘s father. Pleased to meet you!”
* * * *
The vicar had managed to hustle six of us into the vestry, and barricaded
the doors shut behind us. Clarissa‘s father had been deputed to make
sure no one else got in. Clarissa and her mother were comforting
each other, although it already looked as though Clarissa‘s mother had
recovered sufficiently enough to be able to napalm a small village, if
that would help. The vicar made sure he was positioned between the
two women and the bearded man, who was puffing himself up ready for an
argument. Clive was trying to look invisible in a corner.
I felt that my place was with the bridesmaids, who obviously needed
consoling at this stressful time. But the vicar wouldn‘t let me leave.
”Look out for the groom.” He whispered; then motioned for everyone
to be quiet.
”Let‘s hear what this gentleman has to say.” He said, and indicated
that the bearded man should have the floor.
”Thank you sir,” said the bearded man, ”Forgive my colloquial;
English not my first language. I am from Russian family of good repute.”
He sighed. ”Alas, my daughter Olga. Wild child. I
look for ivory tower of learning. She seeks western university of
degenerate social mores and Dangerous Sports Club. Clive‘s university
top of list.”
Clive and I exchanged a quick glance of triumph for our alma mater.
”Yeah!” Olga‘s father looked at us coldly.
”Olga free to lead own life; wastes it participating in dangerous sports.
Seeks ever wilder thrills. Until bungee trip to black hole.”
I shot another glance at Clive. He had been on the bungee jumping
trip to the event horizon of the black hole in M87. What hadn‘t he
told me? But Clarissa‘s mother wasn‘t going to let anyone else run
events for very long.
”This is all very interesting, I‘m sure, and I think we all sympathise
with you over your uncontrollable daughter — I know I do. But what
does this have to do with this wedding?”
”Will explain," said Olga‘s father. ”Russian federation still eyed
with suspicion by authorities in Virgo cluster; refused to grant Olga visa. Almighty tantrum by daughter. But wobbly no good; immigration still
stony-faced.” He paused, shaking his head sadly.
”So what happened??!!” cried Clarissa‘s mother, losing patience.
”I married her,” said Clive, very quietly.
There was dead silence for several seconds. Then Clarissa burst back
into tears and threw herself howling into her mother‘s bosom. Clarissa‘s
mother gave Clive a look that suggested his remaining life would be neither
long nor pleasant.
”I mean,” said Clive, ”it was only to allow Olga to apply for a visa.
I didn‘t love her or anything.” Then he looked at Olga‘s father,
”Oh sorry, didn't mean to...”
”Apology accepted,” he said, ”understand Olga‘s persuasiveness.
Do anything to avoid further tantrums. But marriage certificate still
valid. And marrying second time against law.”
”But,” Clive said, ”she‘s no longer.” Then he too burst
The vicar, who was clearly used to dealing with scenes like these on
a regular basis, had to take charge again and restore order. Eventually,
Clarissa sat by the registry book, clearing her smeared mascara, and Clive,
in the other corner of the vestry, took to blowing his nose loudly.
”Ready to begin again?” Said the vicar. Clive nodded.
”We drew lots to see who‘d make the bungee jump, and who‘d pilot the
two tether space ships. Olga and I won the lots and got to make the bungee
jumps. We dropped out of orbit and prepared to make the pass close
to the black hole event horizon. In our separate spaceships, we each
carried out checks of our own equipment. But, you know, Olga hadn‘t done
this kind of thing very often, and she wasn‘t thorough enough with her
”You see,” said Clive, sniffing loudly, ”Olga‘s jump went wrong.
Maybe she hadn‘t calculated the stresses, maybe she just hadn‘t taken enough
care. But when she jumped, when she fell towards the black hole,
when the bungee rope tightened, and caught round her ankles, to pull her
up to safety, the knot came undone on Olga‘s bungee rope. And she
fell into the black hole.”
There was a shocked silence, for a second or two, as we contemplated
Olga‘s terrible fate. Then Clarissa’s mother looked purposefully and directly
”So she‘s DEAD. And so you are a WIDOWER, are you not?”
She gave Olga‘s father the most dismissive of glances. ”You have
my deepest sympathises. But let‘s get on with the marriage of my
daughter to this . . this idiot.” She gestured angrily at Clive.
I had tried very hard to keep out of the arguments; my only thoughts
were to take care of the ring until it was safely onto Clarissa‘s finger,
and then to take care of the bridesmaids at the reception. But my
conscience kicked in now. I coughed for attention.
”Excuse me. I‘m afraid things aren‘t quite that simple.”
Clarissa‘s mother fixed me with her raptor-like stare. ”What
do you mean, ‘things aren‘t quite that simple?‘ This man‘s daughter
married my prospective son-in-law. The bride fell into a black hole.
So now she‘s dead, and he‘s free to marry my daughter. And frankly,
he had better take more care of her.”
”Sorry to be a nuisance,” I said, feeling myself sinking deeper into
quicksand, ”but there really is a problem. You see, in our frame
of reference, Olga isn‘t dead yet.”
Clarissa‘s mother looked ready to tear my throat out, but fortunately
the vicar was in the way. ”As a man of the cloth,” he said, ”I know
a lot about running weddings, but I‘m not completely au fait with the workings
of black holes. What exactly do you mean by a ‘frame of reference‘?"
”Never mind that!” screamed Clarissa, ”IS THE MINX DEAD OR ISN‘T SHE!?”
I waited for the vicar to restore order. ”As far as we are concerned,”
I said, ”time slows down in the vicinity of the event horizon of a black
hole. Clive found this out when he returned from bungee jumping onto
the event horizon. While only minutes had passed for him, months
had passed in the outside universe. But Clive only approached close
to the event horizon. Poor Olga, after her knot came undone, crossed
that horizon. And as she crossed the event horizon, from our perspective,
time stopped completely for her. In our frame of reference, she is
still crossing the horizon, still alive, and still married to Clive.”
Clarissa burst into tears again. ”Oh mummy, you were right.
Clive‘s just as worthless as you said. I shouldn‘t have let you spend
all that money on the wedding.”
Clarissa‘s mother comforted her. ”Hush, sweetest, we‘re not finished
yet. We might still get that ring on your finger. We might
still get to enjoy the reception daddy paid so much for.” She turned
to the vicar.
”I have no idea if this know-all of a best man has any idea what he
is talking about. Let us assume that he does. It seems to me
that Clive and this Olga woman have been separated by several million light
years for several years. If that doesn‘t constitute grounds for divorce,
what does?” I could see that the vicar sympathised with her reasoning,
but my conscience was still tickling me.
”It isn‘t that simple.” I said, my heart sinking. ”Clive has been separated
from Olga for several years, but, from Olga‘s point of view, she‘s only
been married to Clive for a few days. Remember, she‘s only just reached
the event horizon.”
”Well why don‘t we ask her?” Clarissa‘s mother snarled.
I shook my head.
”In theory, we could communicate with her on the event horizon.
In practice, time has slowed down so much that individual photons now arrive
hours apart. Practically, she has already fallen into the black hole.”
”So she has died in the black hole.” Interjected Clarissa‘s father,
who had been encouraged by the prospect of seeing some return on his money.
”Not yet.” I said. ”In Olga‘s frame of reference, her end came
mercifully quickly.” I gave Olga‘s father a respectful nod.
”About two hours after she crossed the event horizon, in her time, she
reached the singularity of the black hole. Some time before that,
the gravity of the black hole spaghettified her — I‘m sure it must have
been quite a journey. But, from our perspective, she hasn‘t even
entered the black hole yet.”
The vicar was still trying to keep on top of events. ”You tell
us that time flows differently for different people, but it leaves me concerned
for the spiritual well-being of poor Olga. How can God judge her
soul, if she isn‘t even dead in our frame of reference? Surely there
must be a day of reckoning?”
I thought about that one. ”Yes, there is. Eventually the black
hole will evaporate.”
Clarissa‘s father interjected. ”And then we‘ll know she‘s dead
”Absolutely. You won‘t be able to tell her from all the other
quarks in the explosion."
”Great!!" said Clarissa‘s mother. ”So when can we serve the divorce
”The evaporation? That won‘t happen for another hundred or so
I thought that Clarissa‘s mother was going to strangle me, but the
vicar rushed to my defence. ”Just a few more questions. Astrophysically,
do we agree, Olga is still trapped on the event horizon?"
I nodded, and so did the others, grudgingly. The vicar continued.
”Practically, however, she is dead to us." We nodded again.
That stumped us. The vicar nodded sagely. ”I see this kind
of thing all the time. There are some disputes that must be settled
before the courts, and this is one of them. None of us have any real
idea what the law has to say on this matter, or if indeed our legislators
have even considered the circumstances. But I am afraid, ladies and
gentlemen, that I cannot allow Clive and Clarissa to be married today."
Well, I thought he had summed up the situation quite nicely.
Clarissa‘s father began to remove the barricades, so that the decision
could be announced to the congregation. I was about to make my way through
the door when Clarissa‘s mother grabbed my arm in a vice-like grip.
”Where do you think you‘re going?” she demanded of me.
I muttered something about seeing to the bridesmaids.
”Well, young man, you have obviously not paid attention to your book
of wedding etiquette.” She was right there; I had completely ignored
the copy she had thoughtfully provided me with.
”Vicar, come here!" she commanded. Suddenly her husband was steering
the vicar firmly towards his wife and me. I felt things were about
to get out of control.
”Young man,” Clarissa‘s mother said steely, ”on the bride‘s wedding
day, if the groom cannot go through with the ceremony, then there is one
further duty incumbent on the Best Man."
The grip on my arm was getting tighter.
”Now, where did you put that ring?”
* * * *
Reader, I nearly married her.
Fortunately, the vicar was able to convince Clarissa‘s parents that
there was no legal contract forcing the best man to marry the jilted bride;
they accepted his word only grudgingly after much further argument.
During which time the congregation dispersed, and the bridesmaids disappeared
with the photographer and his assistant. Not a good day.
The lawyers we consulted drooled at the anticipation of extensive litigation
over the legal status of poor Olga. The case, they said, would make
legal history, and would certainly progress to the highest court of appeal.
Indeed, they added, it was not entirely certain as to whether or not the
case would reach its completion before the evaporation of the black hole
rendered the final judgment irrelevant. We decided not to proceed
with the case.
Which would have been the end of the story, except for one curious
event a few weeks later. I dropped into a pub in town, not my local,
for a quick drink, and was amazed to see Clive at the bar, enjoying a drink
with Olga‘s father. At least, I think it was Olga‘s father.
By the time I reached the bar, Olga‘s dad was gone, and Clive was about
to leave. He was startled to see me.
”Wasn‘t that Olga‘s father?" I said.
”Who?" said Clive, shiftily.
”WHO??!! You know, the Russian guy whose daughter you married
. . . Remember?”
”Russian guy?” said the barman, absently, ”that was Jim Smith, from
the Dangerous Sports Club. Never been to Russia in his life, far
as I know, but he does a great Russian wild man impersonation. Why
are you looking at me like that, Clive??”
I decided not to enquire any further, and bought a round of drinks
instead. Clive looked at his pint and proposed a toast.
”To knots!” He cried, and raised his glass, ”May you take great
care over tying them..."
”To knots," I countered, ”especially when you cut the Gordian
”Yeah, whatever." said Clive.
But I think he understood what I meant.
X-ray Astronomy - Part 2
The Sources of Radiation
by Paritosh Maulik
In the last issue of MIRA, Number 53, we looked at the various detectors
and methods used to detect X-rays in astronomical observations and how
they differ from optical telescopes, in this article we look at the sources
of the this still mysterious form of emissions.
Before the X-ray sources were confirmed by observations, physicists
suggested that in a binary system of a normal star and a dense companion,
like a black hole or a neutron star, material can exchange from one to
the other and these could be the possible cosmic X-ray sources. There
are basically three types of possible systems:
1) Roche — Lobe Overflow of the Primary
The Low Mass X-ray Binary system: One of the pair is a normal
star is similar to or less than the Sun‘s mass. The point at which
the gravitational pull from each other is equal and opposite is call the
Roche-lobe. If the orbits of the stars is very close, the Roche-lobes
occurs close to the normal star. A funnel like situation develops
and mass can be dragged out from the normal star to the compact star.
An accretion disk will develop around the compact star. All the material
in the disk will not have same angular momentum, so as material in the
disk spirals into the compact object under its strong gravitational pull,
it can heat to a temperature which is high enough for the gas to emit X-rays.
2) High Mass X-ray Binary
In this case the X-ray binary consists of one massive star about 10
times the mass of the sun or greater and the companion is either a neutron
star or a black hole. Radiation from the primary causes gases which
are ionised, ie. in a charged or plasma state, to move out from the primary;
this is called radiation pressure. When the companion star accretes
this plasma, it converts some of the plasma into X-rays.
3) Capture of circumstellar material from a Be star primary
This X-ray binary consists of a Be star and a neutron star. Be
stars are a special type of star which rotate very rapidly and eject a
large amount of matter at random intervals from its equatorial plane.
Strong spectral emission lines of hydrogen and helium are present.
When this material comes under the influence of the secondary star, accretion
takes place and X-rays are generated. But since the emission of matter
occurs at random, the X-ray source is a transient one.
Pulsing and Flashing Binary Stars
A X-ray binary system in which one of the stars is a neutron star.
X-ray emission has been seen to occur either as pulses or in short bursts.
The neutron stars generally have very strong magnetic fields, amongst the
highest in the universe. This magnetic field can cause matter from
the companion binary to funnel into the magnetic pole of the neutron star.
The presence of this strong magnetic field causes the X-rays generated
by this process to be very directional and since the system is rotating,
the net effect is like a beam from a lighthouse. Both of these stars
also rotate on their own axes. When the material falls near to the
magnetic poles of the neutron star, it gives off X-rays and since it is
also rotating on its own axis, it appears as pulses of X-rays.
Although we have said that the magnetic field associated with the neutron
stars is strong, the low mass X-ray binary systems sometimes show a weaker
magnetic field; in these systems, the flow of material is somewhat irregular,
and hence the X-ray emission. This is called Quasi-Periodic
Oscillations. These oscillations originate from the interaction between
the neutron star and the accretion disc or the irregular flow of material.
Study of these systems can give clues of the interaction between the neutron
stars and its surroundings.
In a X-ray binary system, if we can determine the orbital parameters
of the primary star from the Doppler shift, we can calculate the mass of
the secondary. If the mass of the secondary is around 1.4 times
the solar mass, it is a neutron star; if the mass is far higher around
2 solar masses, the companion or the secondary star is a black hole.
Although black holes have a feeling of inevitability about them, in
reality these are the final phase of larger stars above 2 times the solar
mass and shrinking into itself (collapsing). Once it has reached
a critical radius called the Schwarzchld radius, the gravity is so high
that even something travelling at the speed of light can not overcome the
strong gravitational field.
Let us imagine a rocket is fired from the Earth, the energy on the
rocket due to gravitation is given by E = GMm/r2 where G is a constant,
M is the mass of the Earth, m is the mass of the rocket and r is the distance
between the rocket and the centre of the Earth. We can assume this
to be approximately equal to the radius of the Earth. If the velocity
of the rocket is v, its energy is: E = ½ mv2.
The rocket can only escape if its energy is at least the same or greater
than the gravitational energy. Now by solving these two equations
we can work out the minimum velocity needed by the rocket to take off,
i.e. escape. Thus v works out to be v = ( 2GM / r ) ½.
This equation tells us that the higher the gravity, the higher the
escape velocity has to be for the rocket to break free from the gravity.
Let us imagine we fire a spacecraft from the Earth and its velocity is
around 12 km/sec. It would escape Earth‘s gravity, hence escape velocity.
Now if we fire the same spacecraft from another launch pad somewhere with
higher gravity, we would need still higher escape velocity.
The gravity in a black hole is so high that the escape velocity is
higher than the velocity of light, in other words, something travelling
at the speed of light cannot escape, i.e. even light can not escape.
So if a star with a mass like the Sun shrinks to a radius of 3 km, it would
be invisible, this size is called the Schwarzchld radius compared to about
700,000 km, the radius of the Sun now. Schwarzchld radius, R = 2 (G.M)/( c2 ); G gravitational constant, M mass of the star, c velocity
However, there can be massive black holes with masses of about 100m
times the solar mass, and radius of millions of km. These can occur
at the centre of galaxies. Most of the smaller black holes are of
2 to 15 times the solar mass and the radius is of the order of a few km.
As discussed, we can‘t see black holes, but if we can detect a binary
system and work out the mass of one the stars, we can deduce if the companion
star is a black hole. Such a system is expected to occur in Cygnus
X-1. One of the possible candidates for black holes are binary X-ray
sources. From the motion of the binary, the mass of the dwarf companion
can be calculated. Once we know the mass, we can say if it‘s a neutron
star or black hole. As we have seen earlier, if the mass is of the
order of 2+ solar mass, it is likely to be black hole (Oppenheimer -Volkoff
Being a black hole imposes a strong gravitational force on the companion
star and material will fall into the black hole. As the gasses spin
around the black hole, friction causes a sharp rise in temperatures, high
enough to emit X-rays. This accretion of material does not occur
at a steady rate, but at an uneven rate. This leads to variation
of X-ray intensity; also in addition, if the companion star during its
orbit eclipses the black hole, to the observer it appears as periodic emission.
Cygnus X-1, is a highly variable and irregular X-ray source and it
is strong candidate to be a black hole. Its X-ray emission occurs
in bursts of hundredths of a second. In an hundredth of a second
light travels 3000 km. So an object can- not flash faster than the
speed of light, we can expect it takes at least a hundredth of a second
for the X-ray to travel through the dwarf. In other words the size
is of the order of 3000 km; the diameter of the Earth is about 12700 km.
The companion to the blue supergiant star HDE226868, at 30 solar masses,
works out to be about 6 — 15 solar masses. This system has a period
of 5.6 days. A word of caution: the spectral data of HDE2268686 may
also be interpreted as this star not being so heavy, which in turn bring
the mass of the Cygnus X-1 to about 3 solar mass i.e. close
to a neutron star.
Orbit and Mass of Binary Stars
By determining the orbital and Doppler shift parameters we can calculate
the mass of the companion secondary star in a binary system. In a
X-ray binary system, if the mass of the binary is of the order of
about 1.4 solar mass, it is a neutron star; if the mass is greater than
2 solar mass, the companion star is a black hole and thus validating the
It has been suggested that the total mass of the universe is 10 to
20 times the visible mass. The bulk of the visible mass was created
during the big bang explosion and this invisible mass consists of fundamental
subatomic particles. Collectively this is known as baryonic matter.
The possible existence of the dark matter was estimated from the amount
of light elements like hydrogen and helium formed during the big bang explosion.
The properties of high temperature gas in clusters of galaxies and the
high speed of movement of galaxies in this gas cloud, temperatures may
be as high as 108 K, hot enough to generate X-rays.
By determining the X-ray temperature, and gas characteristics, in clusters
of galaxies, group of galaxies or a massive elliptical galaxy one can determine
the mass. The analysis of the data show that the mass of the gas
is around 3 — 5 times the mass of the stars and the baryoinic matter is the
around 10 — 30% of the total mass in stars, in other words, the X-ray studies
have shown that the dark baryonic matter can extend far beyond the visible
limit of the galaxies.
Types of Supernovae
There are two types of supernovae
I) In a binary system of a red giant and a white dwarf, mass
flows from the red giant to the white dwarf, the white dwarf can not support
itself and collapses or implodes. These types of supernovae do not
show hydrogen spectral line.
II) At the end of the life cycle of a massive star the nuclear
synthesis comes to a near end, it can not support itself when the core
transforms to iron. If the mass is large enough, it will collapse
to become a supernovae. The massive stars are generally made of hydrogen
in their atmosphere. So during the supernovae explosion hydrogen
spectral lines can be seen. But if the star is too massive, the stellar
wind may blow off all the hydrogen and a hydrogen line may not be present.
White dwarfs are generally free from hydrogen and other elements.
A word of caution: a type II supernovae was once termed as Ib, on the basis
of hydrogen emission characteristics, before the physical processes involved
were fully understood.
The core of an extremely large star is very small compared to its overall
size. The nuclear synthesis going on in the star over millions of
years gives rise to formation of elements heavier than hydrogen and helium.
The supernovae explosion causes these elements to disperse into the interstellar
space. The star becomes a supernovae remnant.
The nuclear energy of a stable star is enough to synthesise elements
as heavy as iron. In order to make still heavier elements we need
still higher energy. The supernovae explosion can provide this additional
energy. This is how the different elements seen today are formed.
Supernovae explosion does not occur regularly, when they do, they are
examined thoroughly. In earlier times these were reported as appearing
and disappearance of new stars.
The brightness of some stars goes through a periodic variation.
This period is very regular, for example U Gem, the optical intensity reaches
to a maximum of about 100 fold in around 120 days and then falls back to
the original level in about 10 days. These are called Cataclysmic
Variables or CV‘s. It is now known that these are binary systems,
in which the primary is a white dwarf, the end phase of star; its nuclear
reaction has ceased, its outer part has blown away; has a maximum mass
of 1.44 solar mass, i.e the Chandrashekhar limit. Under such conditions
the fundamental particles do not behave normally, the star stops collapsing,
its surface temperature is greater than 10000°K. The star cools
slowly becoming fainter and redder. The secondary is a sun like star.
The gravitational force of the primary pulls matter on to the primary and
the release of this gravitational energy is seen as a energy burst.
Cv‘s were first observed by the optical astronomers and they classified
the CV‘s according to the nature of the outburst. Classical novae
are those where the outburst occurs once and the intensity is very high.
In white dwarf the fuel has run out and on receiving fresh fuel mainly
hydrogen by the accretion process, reaction can start again.
Dwarf novae outbursts are smaller in amplitude but higher in frequency;
these may appear to be similar to classical novae outbursts, but
the main difference is there is a temporary increase in the rate of accretion
onto the primary.
X-ray detectors on board the Uhru and Einstein missions suggested that
many of the CV‘s are weak X-ray sources. The material leaving the
secondary cannot fall straight into the primary. The
material forms a disc ™ an accretion disc — around the primary. The
friction within this layer heats up the material and it slowly spirals
into the primary. Once the disc hits the surface of the primary the
temperature can go up as high as 106°K — high enough to generate X-rays.
The surface of the primary is not a well defined and is now currently being
It now appears that the majority of the CV‘s with strong X-ray
emission have very powerful magnetic fields associated with a white dwarf.
The magnetic field can be far more powerful than the Earth‘s magnetic field
and can alter the shape of the accretion disc and produces stronger X-ray
CV‘s with nuclear fusion rather than just accretion is also possible.
This is the case with classical nova outbursts. X-ray astronomy has
revealed another group of objects, Super Soft Sources. The energy
output from these sources is somewhat low about 0.5 keV. This suggests
the surface temperature to be about 20000°K to 80000°K. The
mass of the white dwarf is possible greater than that of the Sun.
Theoretical calculations suggest that continuous nuclear reaction on the
surface of a SSS may eventually lead to a supernovae explosion.
Approximately once a day there occurs sudden bursts of very powerful
gamma rays which soon after the outburst, disappear. This phenomenon
appears to occur at random and very little is known about this source.
It was once thought that the neutron stars within the Milky way was
the sources of the gamma-ray bursts. But this idea is being questioned.
Any successful model should be able to explain these very high energies
and the emission mechanism based on our present knowledge. There
appears to be two basic ideas
I) Sources are in the Milky Way: once it was thought that the
neutron starts have a velocity of 200 km/s; now it appears that these can
have velocity greater than 500 km/s. If this the case, then, these
stars can move into a halo or corona around the galaxy at a distance of
about 300 ly. Corona at such a distance may contain enough sources
and from a distance such as Earth the distribution as appears to be uniform.
II) The sources are outside the Milky Way: The study of the other
components of the electromagnetic spectrum associated with the GRB sources
suggests that the majority of the these lie outside the Milky Way.
The cosmological model implies homogeneity, any inhomogeneity is perhaps
due to redshift effect. This model suggest that the energy release
from this burst is around 1053 to 1054 erg. Similar level of energy
release also takes place during supernovae explosions, but in a supernovae
explosions, kinetic energy of the dust and gas cloud and visible light
account for about 1050 erg of the energy release and only a small fraction
is associated with the X-ray energy release. In the case of GRBs
almost all of the energy release is in the X-ray / gamma-ray range.
Active Galaxies and Quasars
On the average, the total weight of a galaxy is about 1110 times solar
mass stars; + 110 times solar mass interstellar gas; + 1012 times solar
mass dark matter. But the calculations show that a large number of
these, may be as high as 50% heavier. It now suggested that long
long ago when the galaxies were younger, the stars in the core were closely
packed. This led to collisions among stars; the stars merged and formed Massive Black Holes (MBH), with masses as heavy as 106 to 109 times solar mass. As discussed earlier, in a X-ray binary star system, an accretion disc forms and this produces a very large amount of radiation ranging from infrared to gamma-rays. This MBH and the accretion disc, is responsible for what is now termed as Active Galactic Nuclei (AGN).
Our own galaxy the Milky Way may have an AGN, but it appears to be somewhat quiet, and is not very prominent. Seyfert galaxies (see later) have moderately mass AGN while the accretion in the quasi-stellar objects with high-mass AGN , makes the quasi-stellar objects very luminous.
In about 10% of the AGN, the MBH and the accretion disc produce narrow beams of energetic particles and magnetic fields. These are ejected in opposite directions away from the disc. The radio jets travel almost at the speed of light and can spreads outwards. Radio galaxies, quasars and blazers (see later) are in fact AGN with strong jets. The way these AGN appear to us depends on their orientations.
In the 1940‘s Carl Seyfert photographed the galaxy NGC4151 and observed
that it has a very bright point like nucleus. Its spectrum is somewhat
unusual, in addition to the continuous spectrum and absorption lines like
stars, it also contain powerful emission lines. Some of these lines
are common, like hydrogen lines, but some are unusual, such as twice ionised
oxygen lines. It is suggested that the gas is at very high temperature.
The broad spectral line is explained as Doppler shift in all directions,
at a speed around 2000 km/s. The nucleus brightness can vary in a
time period of months; this suggests the nucleus size to be less than 3
ly. The total luminosity can be equivalent to about 1010 times our
It is now believed that the Seyfert galaxies have a massive black hole
in their centres. Around this MBH an accretion disc forms; the radiation
raises the energy of the gas, its temperature rises and forms unusual spectral
lines. Seyfert galaxies may be the lower -luminosity examples of
the quasar activity. The majority of the Seyfert galaxies are either
spiral or barred spiral galaxies.
Radio observation have shown that some of the galaxies can emit powerful
radio energy, as high as 1038 W. The nucleus of these galaxies also
emit strong radio energy and these nucleus appears to be at the centre
of the observable galaxy. Jets stream out from the centre in two
opposite directions into the interstellar medium. Additionally a
pair of lobes can de detected. These lobes stretch outside the visible
limit of the galaxy. At the centre of these galaxies are huge sources
of electrons and protons with high energy. In the presence of a strong
magnetic field, these particles can accelerate; this process of acceleration
can cause a release of the energy of the particles by electromagnetic radiation
at speed of light. This is termed as synchrotron radiation and is
now the acceptable mechanism for the generation of the radio and X-ray
These are radio sources and their output varies over very short periods,
months or days. A galaxy or a radio galaxy does not vary its output
over such a short period. The spectral pattern of these are somewhat
different from stars and galaxies. The spectral lines are highly
red shifted, which suggests that quasars are very far from us and formed
Balzars are highly variable quasars and star like objects with AGN,
the brightness varies over days, the emissions are polarised, high speed
jets of plasma and radiation are also seen to be emitted from these objects.
If one of its jets point towards the Earth, a relativistic effect occurs
when the velocity of a particle approaches that of light; its rest mass
increases and if the particle is charged, it emits electromagnetic radiation
in a narrow beam. The higher the velocity of particles, the narrower
is the beam.
Blazers emit a wide range of frequencies, ranging from gamma rays to
radio. The radio signal is powerful, and occurs over a wide range
of frequencies. The variability in the optical range, over days is
a very important characteristic of balzars.
This group of objects can be classified into two subgroups, 1)
Flat Spectrum Radio Quasars: strong broad emission lines, 2) BL Lac
objects: near-featureless optical spectrum, may be associated with a compact
radio source. About 200 BL Lac objects are now known.
Soft X-ray diffuse background (SXRB) is general low energy 0.1 - 2.5
keV (0.5 - 1.24 nm, wavelength). X-ray background radiation, not
strictly associated with any given source. One of the common X-ray
sources is hot plasma at 106 K. This gives rise to X-rays in the
lowest level 0.1 - 0.2 keV. This hot plasma consists of two parts;
i) contained in the disc of the galaxy, surrounding the Sun, but
it was not made by the Sun. This disc extends from about 150 to 600
ly and is not uniform in size. ii) general distribution in the halo
of the galaxy.
X-ray from the extra-galactic sources such as AGN and quasars, also
contribute to the back ground X-ray radiation. The exact sources
may remain unresolved. The total energy level is higher, greater
than 1 keV.
Instruments abroad rockets in the late 1960‘s first detected SXRB of
the order of about 0.25 keV. Although the resolving power of these
early instruments were limited, it became clear that the X-ray intensity
was greater along the galactic latitude than plane of the galaxy.
This observation was interpreted as the origin of the X-ray is extra galactic
in nature, and the neutral interstellar medium (ISM) in the galactic plane
absorbs the X-ray.
However there were some reports of high intensity X-ray distribution
along the galactic plane and this distribution is non-uniform. But
with further refinement of observations and theory, it was accepted that
the X-rays detected along the galactic plane are also cosmic, i.e.
extra-galactic in nature. Soon it became apparent that a model had
to be developed to take into the account the non-uniform absorption of
X-rays. For example, the X-ray energy distribution in the range of
0.75 keV is more or less uniform, while that in the range of 0.25 keV occurred
in a band along the galactic plane. Several theories were put forward.
By studying other energy levels for the past 20 years the nature of
the ISM became somewhat clearer. It was found that there is localised
deficiency of the ISM in the galactic disc, as if there is a cavity around
the Sun, where there is very little neutral hydrogen atoms; however this
cavity was not related to the Sun. There are other localised zones
ranging from 3 to 15 ly, where there is a drop in the hydrogen level.
In addition, the low hydrogen areas surrounding the Sun also contained
ionised hydrogen, the later appear to occur perhaps in one direction.
It is now believed that the areas of low hydrogen (often termed a cavity)
are the areas where there is hot plasma and this plasma gives rise to the
SXBR in the galactic plane.
Infrared spectroscopy as added further to the understanding of the
hydrogen cavity. It turned out to be narrow (about 300 ly) and extends
outward from the plane of the galaxy. As described earlier the hot
plasma around 106 K within the hydrogen cavity gives rises to the SXBR
in the range of 0.25 keV. This now called the displacement model.
The origin of the hot plasma is not known for certain. The current
thinking is that the cavity existed in the galactic disc, and around 105
years ago, there was supernova explosion, which caused the plasma to heat
There are other sources of SXRB, such as supernova remnants
which spreads radiation over wider angular distribution. The presence
of the hot plasma is now conclusively proven, but refined observations
indicate that the 0.24 keV can exit outside the ISM in the galactic disc
and if this is the case the question is how and why?
A supernova remnant (SNR) is the leftover from a cosmic firework, a
supernova explosion. Such explosions heat up the interstellar medium,
distribute heavy elements throughout the galaxy, and accelerate cosmic
Classification of SNR
i) Shell type remnant: The explosion shock wave expands
like a ripples in a pond. It stirs up the interstellar material in
space, heating the material as the shock wave progresses in the shape of
a shell. Viewing from an edge, the shell appears like a ring and
there is more hot gas in the line of sight compared to looking through
the middle. This is called limb brightening.
ii) Crab-like remnants: These are also similar to the shell
type of SNR, except, that there is a pulsar in the middle throwing out
jets of fast moving materials. These look more like blobs rather
than a ring. Sometimes these SNR are called plerions.
iii) Composite SNR: The appearance depends on the observer,
to put it another way the selected wavelength of the viewing instrument;
under certain wavelengths, the image may look shell like or crab like.
There are two types of composite remnants, thermal and plerionic.
Thermal composites, when studied in the radio waveband, the SNRs appear
as shell type. The radio wave is generated by synchrotron radiation.
In the X-ray range, the same object looks crab like. The X-ray spectra
have spectral lines; this suggests the presence of hot gas.
Plerionic composites, crab like in the radio and X-ray range and a
shell is present. The X-ray spectra in the centre show spectral line,
but near the shell a broad range, no distinctive peaks.
The Age of the SNR
Early historical data, both cave paintings and written documents record
the arrival of ?guest stars‘, these may go back to several thousand years
and we need to confirm this data by independent means. An early attempt
was to determine the temperature of the hot gas from the X-ray spectroscopy.
From this calculation the shock wave velocity can be calculated and thus
the age. This method is simple, but there are other interactive processes
taking place, which can heat up or cool down the gas and these are not
dependent on the shock wave.
For the youngest SNR‘s the following method work well. The SNR
expansion is measured as a function of time and since rate x time = distance,
if we know the time and distance, we can find the rate of expansion.
As an example, let us assume that, over 20 year period the supernova has
expanded by 50%. 20 years ago the size was 100 and now the size is
150, an increase of 50%. In 1 year the size increases by 50/20 = 2.5%.
100% expansion occurs in 100/2.5 = 400 years. To be on the safe side
we say that the explosion occurred less than 400 years ago and since then
it has slowed down.
Why are Supernova Remnants Important to us: its Effect of SNR on the
The universe is rich in hydrogen (about 73%) and about 25% helium with
about 2% other heavier elements. Synthesis of the heavier elements
occur in the stars and the through the Supernova remnants these elements
eventually find their way into planets like Earth. A gas disc in
the interstellar medium (ISM) may enrich to a high concentration in localised
area, such as the spiral arm of the galaxies, for example, in the Milky
Way. This through a process of gravitational collapse may set
in motion star formation. It is the SNR who mix the newly formed
elements into the ISM.
The shock waves also accelerate the electrons, protons and the ions
close to the speed of light through the Fermi acceleration process.
It is believed that a part of the ISM in our galaxy got entrapped into
a supernova shock wave and the excess energy of these particles are responsible
for the cosmic rays. Since these particles are entrapped, the cosmic
rays keeps on occurring. These high energy particles shower
the Earth, but once these strike air molecules, it produces a secondary
cosmic ray shower, consisting of fundamental particles, photons and
gamma-rays. The theory is not complete yet.
Essentially there are three stages of SNR life.
i) The shock wave interacts with the surrounding ISM; the temperature
within the SNR is constant and the shell expands with a constant velocity.
ii) The SNR is now beginning to decelerate and cool. The
products of the supernova explosion including the heavier elements slowly
get mixed into the interstellar gas. The magnetic fluids inside the
SNR shell becomes stronger.
iii) Snow-plough or Radiative Phase: The temperature is now is
around 106 K,; the SNR radiates energy. As it radiates it cools faster,
it shrinks and becomes denser. As it shrinks it cools faster still,
because the material mixes well and looses heat. Now it forms a thin
shell and radiates most of its energy in the optical range. The velocity
still slowing down, the outward expansions stops and the material inside
starts to collapse under its own gravity. After millions of years
the the core of the SNR will break out of the outer shell and will absorb
into the interstellar medium.
More stories from Pam Draper
Thinking of doing a course in astronomy? I thought members might like to see a sample of the questions from the University of Lancashire Cosmology course.
1/ Calculate the age of the universe in seconds when the radius is:
a) the same as the Bohr radius
b) 1 meter
c) the size of the Earth
d) the size of the Sun
What would be the redshift at these times?
2/ Why do we believe that the observed redshift to distance galaxies is cosmological rather than due to the proper motions of the galaxies?
3/ Calculate the rest mass energy for each of a proton, neutron and a electron from the published masses.
In equilibrium conditions ½KT of energy is associated with each degree of freedom.
Assuming there are 3 degrees of freedom for each particle.
Calculate the temperature below which each of these three particles will fall out of equilibrium with its surroundings?
What would be the wavelengths of the photons having these energies?
In what spectral region would these be located?
4/ Explain why the neutrino background radiation and the microwave background radiation have different temperatures today.
At what times and redshift did they have the same temperature?
5/ Suppose that a group of cosmologists want to build a new Space Telescope that can measure the parallax of all the stars in the Milky Way galaxy.
Calculate the resolution that the telescope would require and compare it with our existing telescopes.
Is this realistic?
6/ Explain what the Critical Density of the universe means.
Supposing that the Hubble Constant is observed to be 65km5-1 MPC.
Calculate the critical density of the universe based on this observation.
7/ Two galaxies were 2·5 MPC apart in the universe when the redshift was z=3.
Calculate how far apart the galaxies will be today and at redshift z=1.
8/ List the mediating particles for each of the Fundamental Forces.
9/ Calculate the volume of the universe ( in terms of the present volume Vo ) at redshift z=2 and z=1500.
At what time in the history of the universe would the mean density be the same as the density of water?
What value have you used for the present density?
10/ Summarise the observations that allow us to deduce that we live in an expanding universe?
11/ Explain why it is difficult to make observations or measurements to derive the Hubble Constant?
12/ Do we live in an open, closed or critical universe?
How would we confirm this observational?
Do the theoretical models of the universe provide any clues and do these agree with the current observations?
Gets you thinking doesn‘t it!
Send your answers to these questions by return to Pam, not me! Ed.