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Skywatching

Our neighbouring black hole

Understanding black holes

Black holes are rather menacing objects.

Stars, planets, dust and any other material getting too close fall in. There is an intense flash of radiation and nothing comes back out. There are black holes with masses millions of times that of the sun at the centres of galaxies like ours. There are lots of less massive and much smaller black holes around, formed during the final collapse and explosion of dying massive stars.

Astronomers believe there are about 100 million of these scattered around our galaxy. This suggests there could be one or two quite close to us. In fact, one has just been discovered. It lies around 1600 light-years away. That is close. Some of the stars we see in the night sky with our unaided eyes are more distant than that.

This new discovery has not been seen directly; black holes are small and black, invisible against the black background sky. We are forced to use indirect methods in the search. Black holes are probably the most bizarre objects in nature. However, the recipe for making them is simple: a mass of material, gravity, and some initial compression to start the shrinkage.

The strength of the gravity on the surface of a body depends on two things: its mass and its size. For a given mass, the smaller the size the stronger the gravitational force trying to make the body shrink. If we compress something enough, its gravity becomes stronger than the body's ability to resist compression, so it shrinks. As the shrinkage proceeds the gravity gets stronger, and the shrinkage continues.

Our current understanding, derived from observation of other things in the universe and what we can do in the lab does not tell us where this runaway shrinkage will end. At some point in the shrinkage the gravity at the surface of the body becomes so intense that not even light can get out. If this is the case, then how can we find these objects?

One method is to look at the X-ray, light and other radiation given off by material as it spirals in and disappears into the hole. The radio images we have of black holes show a dark, roughly central area surrounded by a glowing ring. This method works when the black hole is consuming nearby stars, planets and other material.

However, there are many black holes that are not "feeding" and not producing observable radiation. These are referred to as "dormant" black holes. In this case we search for stars that are orbiting objects that have the right masses to be black holes but are otherwise invisible.

We see stars in the core of our galaxy in close orbits around the central black hole. By measuring these orbits we can estimate the mass of the object they are orbiting. This works on a smaller scale too. Many stars are members of multiple star systems, with two or more stars born together and staying together, orbiting around each other.

If one star of one of these systems is invisible, perhaps because it has become a black hole, its presence and mass can be determined by analyzing the orbits of its visible siblings. That is how this nearby black hole was discovered.

Astronomers discovered a star that was orbiting around something invisible. After careful observations using different astronomical instruments the unseen object was identified as a black hole. However, there is still a problem. The star orbiting with the black hole is a sun-like star.

It appears that two stars were born together; one was much more massive than the other. The massive one shone very brightly for a few million years and then blew up, ending up as a black hole.

The other star had a mass similar to that of the Sun. Such stars survive for billions of years. It must have been around when the massive star blew up. Since the stars lie close together, about the distance between the Sun and Earth, the sun-like star should have been destroyed.

How it survived the explosion is a major puzzle.

•••

After sunset, Jupiter lies in the south-east and Saturn in the south. Mars rises later. The Moon will reach its first quarter on Nov. 30.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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Learning more about 'white dwarf' stars

'White dwarf' stopwatches

It might be surprising that “white dwarf” stars, the relics of stars like our Sun that have run out of fuel, are useful cosmic stopwatches.

The resulting objects are about the size of the Earth, but still with almost the mass of the star. They are white hot, around 10,000 C, which is why these stars are known as white dwarfs.

With no source of energy, their fate is to radiate away all their heat, ending up as cold, dark cinders. However, this takes an extremely long time.

Assuming a white dwarf started off with a temperature of around 10,000 C, we can measure what that temperature has fallen to now and calculate how long ago that star became a white dwarf. That means we can estimate the ages of things containing white dwarf stars.

For example, the Hubble Space Telescope has detected many white dwarf stars in the central regions of our galaxy. These stars are around 13 billion years old, confirming other estimates of the age of the Milky Way—13.6 billion years.

A lot of astronomical attention is being paid to a pair of recently discovered white dwarf stars, poetically named WD J2147-4035 and WD J1922+0233, which have cooled to about 3,000 C and 3,300 C respectively.

From that, we estimate the first of those stars became a white dwarf around 10 billion years ago and the second roughly nine billion years ago.

If the stars that became these white dwarfs were like the Sun, then those stars would have formed a few billion years earlier, which brings them fairly close to the beginning of the universe.

This is particularly intriguing because it looks as though these stars had planet and because the light from those white dwarfs carries the signatures of elements such as potassium, calcium and lithium.

The only way they could have collected them is through the debris of destroyed planets falling onto their surfaces. The evidence is subtle, but detecting it was made easier because, unlike those white dwarfs the HST observed around the centre of our galaxy some 26,000 light years away, this pair of stars lie close by, around 100 light years away.

That means their light is strong enough for subtle indications of elements to be detected, with no confusion with the light from other stars nearby.

Galaxies form through the coalescence of smaller galaxies. During that process collisions between gas clouds stimulate the formation of lots of stars. Some of those stars would have had masses many times that of the Sun, and would have blown themselves up within a few million years.

The lower-mass stars became white dwarfs, which is why they can tell us the age of the Milky Way. However, it is really intriguing that at least two white dwarfs from stars that must have been among the first in our galaxy had planets.

To make planets you need elements like silicon, oxygen, iron, phosphorus and all the others. These did not exist when the universe began, just under 14 billion years ago.

The only elements available were hydrogen and helium. Luckily these are all you need to make stars. All the other elements are produced as waste products by the process of energy production in the stars, and distributed into space when these stars exploded at the ends of their lives.

We believe those early stars were supermassive, blue giants, which had very short lives—maybe a million years or two before exploding. That the two white dwarfs formed nine and 10 billion years ago, from sunlike stars with planets, suggests the raw materials needed for making planets existed soon after the first stars formed.

There is a downside to observing white dwarfs. Because they ration their energy radiation over many billions of years, they are incredibly dim.

•••

After sunset, Jupiter lies in the south-east and Saturn in the south. Mars rises later. The Moon will be new on Nov. 23.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



Learning from impacts areas on the Moon

What the craters tell us

If you look at the Moon through binoculars or a small telescope, two sorts of terrain stand out.

There are lighter-coloured, mountainous regions that are heavily cratered. They are so heavily cratered that it is hard for an incoming object to make a new crater without hitting an existing one.

Surfaces like this are called "saturated". The plains are called "maria" because early astronomers thought the Moon had seas. They have romantic names like Mare Tranquillitatis (Sea of Tranquillity), Mare Procellarum (Sea of Storms), Mare Nubium (Sea of Clouds) Mare Crisium (Sea of Crises) and so on. The maria are not water, but ancient lava flows.

At various points in its history the Moon has been hit by large objects, which blasted huge craters. Lava flowed up to fill the craters and then overflowed onto the surrounding land. We can see craters that are partially buried by this lava. The poetically named "Sinus Iridum" (Bay of Rainbows), is defined by a crescent of mountains that are the rim of a big crater that was buried by the lava forming the Mare Imbrium (Sea of Rains). However, one thing about these lava plains stands out, they are far less heavily cratered than the mountainous areas, suggesting that when the maria formed, the frequency of impacts had decreased a lot.

The Apollo astronauts brought back lots of rock samples from the Moon. Some of them were from the mountainous areas and some were from the lava plains. Using radioisotope dating it was possible to determine when the rocks solidified— in other words, when the rock formed.

As expected, the rocks from the maria were younger than the rocks from the mountainous areas, but not hugely younger. Like the Earth, the Moon formed about 4.5 billion years ago. The mountainous terrain dates back some four billion years. The impacts that formed the other maria are estimated to have taken place between three and 3.5 billion years ago.

The relatively light cratering of the maria indicates that by the time they formed, most of the bombardments that moulded the heavily cratered mountainous terrain happened within half a billion to a billion years after the Moon formed. The bombardment has not stopped and impacts are still happening, but at a far lower rate.

This fits our ideas as to how the Solar System was formed. A collapsing cloud of dust formed lumps. These lumps grew by colliding and sticking together. The biggest lump formed the Sun, and other lumps formed the planets.

As the newly born planets orbited the Sun, they swept up the material sharing their orbits. When the orbits had been "swept clean", the impacts became much less frequent, but did not stop completely. Even today there are objects crossing the Earth and Moon's path around the Sun, many of which pose a collision risk. These orbit-crossing bodies are a continuing threat, because objects in safe orbits continue to be moved into dangerous orbits by the gravitational attraction of the planet Jupiter.

So, by about 3.5 billion years ago, the big bombardment had ended. That was fortunate for us on Earth, because along with the other planets, we were being hit at least as often as the Moon was. The geological record here on Earth suggests that life appeared after the worst of the bombardment was over, and conditions were stable enough for living creatures to survive, proliferate and develop. We don't know if there were any false starts.

In an age of huge telescopes, looking further out into the cosmos and back to the beginning of the universe, it is intriguing that important information on how our world began is written on the face of a familiar object that lights up our night skies.

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• After sunset, Jupiter lies in the south-east and Saturn in the south. Mars rises later.

• The Moon will reach its last quarter on Nov.16.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



193945


Stars absorbing planets while leaving 'zombie' planets behind

'Zombie' planets

It is widely accepted among the scientific community that over the next few billion years the Sun will swell into a red giant star, absorbing Mercury and Venus, and cauterizing the Earth.

Then the Sun will sneeze off its outer layers, becoming a white dwarf star, which will be so miserly with its energy output that the remnant of the Earth will wind up as an airless, waterless, deep-frozen rock ball. More massive stars collapse and explode. These explosions are so huge that any planets those stars have will almost certainly be melted and then vaporized.

It is therefore really surprising to see planets that were seemingly at point-blank range, and should be dead and gone, but are still there— "zombie” planets.

Around 2,300 light years away lies the pulsar PSR1257+12. This is a neutron star, the remnant of a massive star after it underwent a supernova explosion.

This neutron star is rotating 161 times a second. Spinning at this speed is the result of a big star shrinking into a relatively tiny neutron star. For example, if we shrunk the Sun down to 10 km, a typical neutron star diameter, it would be spinning almost 50 times a second. The star that became PSR1257+12 was bigger than the Sun, leading to a faster-rotating neutron star. The process involved here is what we see when a spinning skater pulls in her arms, making her spin faster.

There are beams of radio emission coming from PSR1257+12, rather like beams of light from a lighthouse. We receive a pulse of radio emission every time the beam points in our direction.

The rotation rate of something the mass of a star is going to be very stable, which means the timing of the pulses we receive can be precisely timed. In the case of this particular pulsar, the pulse timing varies slightly but measurably, in a cyclic pattern, showing the neutron star has a tiny wobble.

If you have a ball on the end of a piece of string and are whirling it around your head, others would see you wobble, and guess what you are up to, even though they might not be able to see the ball. PSR1257+12's wobble indicates it has three planets orbiting around it.

Getting into the Hallowe'en spirit, the pulsar has been nicknamed "Lich", after an undead creature in western folklore. Moving outwards from Lich, the planets are Draugh (an undead creature from Norse mythology), Poltergeist (a mean-minded spirit who likes to bang doors, knock things over and move furniture), and Phobetor (bringer of nightmares).

Draugh is small, about twice the mass of the Moon. The other two planets have masses about four times the mass of the Earth. What is surprising is how close they are to Lich. Their orbital periods are only 25, 67 and 98 days respectively. In the spirit of the thing, NASA produced a very unusual poster on the subject describing this strange planetary system. It is fun and very informative.

If those planets were orbiting that close when the giant star exploded and turned into a neutron star, they should not exist now.

The explosion should have vaporized them. It would be easy to conclude the observations are wrong. However, since this system was discovered, back in 1991, others have been found.

One theory is that the planets did not exist before the explosion. The star that exploded had a companion star orbiting around it. Such double stars are quite common.

The companion star was destroyed in the explosion. Most of its material was blasted off into space. The planets formed from the remains.

Those planets seem to have temperatures suitable for life, but orbiting close to a neutron star, in an environment loaded with X-rays and other radiation makes it unlikely.

•••

• In the early evening Jupiter lies in the south-east and Saturn in the southern sky. Mars rises later.

• The Moon will be full on Nov. 8 and will reach last quarter on Nov. 16.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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