What is our galaxy?
SITTING inside the Milky Way, it is hard to see the wood for the trees. Interstellar dust chokes much of the galaxy and obscures some regions completely, but gradually the overall geography has emerged from the mist. Over the past few decades, astronomers have painstakingly mapped the stars in our galactic island to try to figure out its structure. It has been an arduous task, but one that has paid huge rewards.
We now know that the Milky Way is a dense disc of stars, gas and dust some 100,000 light years wide. A bulge of stars about 12,000 light years wide protrudes from either side of the disc’s centre, making it look like two fried eggs back to back. A faint spherical halo of sparse old stars and tight-knit balls of stars called globular clusters surrounds the whole galaxy. In total, the Milky Way contains at least 250 billion stars, possibly as many as a trillion.
This picture began to emerge in 1923 when American astronomer Edwin Hubble became the first person to pinpoint the distances of stars in a distant, fuzzy patch of light called the Andromeda nebula. His measurements revealed that the “nebula” is in fact an isolated cosmic island of stars. That suggested the Milky Way too is a galactic island.
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Observations since then have also shown that our galaxy has bright spiral arms composed of dense concentrations of stars swirling through the disc. We can’t travel outside the Milky Way to see what it would look like from afar, but it may well look like a giant Catherine wheel in space. Such large spiral galaxies are unusual. The vast majority of galaxies are small, faint and blob-like, but ours is magnificent – one of the grandest jewel boxes in the cosmos.
However, our picture of the galaxy is still patchy. “It is frightening how little we know about the structure of the Milky Way, despite how much we have studied it,” says Naomi McClure-Griffiths at the Australia Telescope National Facility in Epping, New South Wales. “Our methods for unravelling the structure are becoming increasingly clever, but there is still a lot to do.”
Swirling spirals
FOR some 50 years, astronomers have known that the Milky Way has a spiral structure, in which bands of bright young stars light up intricate swirls in the disc. But the spiral shapes don’t reflect the paths of these stars, which actually follow circular orbits around the galactic centre.
Instead, scientists believe that some kind of pressure disturbance called a density wave creates the pattern. Orbiting stars and gas clouds periodically pass into spiral pressure waves that stretch from the galactic centre to the edge of the disc, like cars running into a band of denser traffic. The waves compress gas and trigger the formation of copious bright new stars. However, there is a glaring gap in the theory: astronomers admit they don’t know what triggers the density waves in the first place.
In our own galaxy, the spiral pattern is quite hard to see because it is edge-on and the dusty disc obscures much of our view. Astronomers have done their best to map the positions of the bright stars inside the spiral arms, and radio telescopes have picked up signals from the compressed hydrogen clouds inside them. In this way they’ve traced fragments of four main arms named Scutum-Crux, Sagittarius-Carina, Perseus and Outer. There also seem to be a few smaller arcs between them.
In late 2003, a team led by Naomi McClure-Griffiths of the Australia Telescope National Facility announced the discovery of another arm fragment. Using the 64-metre Parkes radio telescope that relayed TV coverage of the first men on the moon, as well as a separate array of six 22-metre radio dishes, her team spied an arc of dense hydrogen about 77,000 light years long running along the galaxy’s outermost edge.
Viewed from Earth, the arc appears nearly 150 times as wide as the full moon and is probably part of the galaxy’s Outer arm. “It’s not exactly like it was hiding. And yet we had never really noticed it before,” says McClure-Griffiths. She points out that we still have a lot to learn about the spiral structure – and we might not like what we eventually find. “We like to think that we live in a magnificent, grand-design spiral,” she says. “But my suspicion is that the Milky Way is a bit more of a mess than we’d like.”
Location, location, location
IF GALACTIC real-estate agents existed, they’d be keen to market the solar system: a desirable property in a quiet area, with excellent views, rich in chemical commodities, and no trouble from noisy stellar neighbours.
The sun sits in a suburb about 26,000 light years from the galactic centre – just over halfway out. Our suburban locale is probably no accident. Scientists argue that in order for life to evolve on a planet, it probably needs lots of elements heavier than hydrogen and helium to create molecules with diverse biological functions, and these heavy elements are most abundant towards the centre of the galaxy. So life-forming planetary systems can’t be too far from the galactic core. On the other hand, advanced life has taken billions of years to evolve, so the region needs to be far enough from the galactic centre to avoid being rocked by violent supernova explosions that regularly occur there.
A team led by Charles Lineweaver at the University of New South Wales in Sydney, Australia, showed last year that the habitable middle ground may be surprisingly narrow. Their calculations hint that advanced life could only evolve in a ring between 22,000 and 30,000 light years from the galactic centre, representing just a tenth of the total area of the disc (Science, vol 303, p 59).
Black holes galore
WITHOUT doubt the Milky Way’s weirdest inhabitants are black holes. For centuries there has been speculation about the idea of objects so dense that their gravity would prevent even light from escaping their clutches, forming rifts in space-time called black holes. But it was only in the 20th century that scientists became convinced they could really form, for instance when a very massive star explodes at the end of its life, leaving a core three or more times as heavy as the sun. No known force can prevent the core collapsing into a black hole.
What’s more, black holes should be common in the Milky Way, where there are typically one or two supernova explosions each century, and a large proportion of these should form black holes. “There should be about 10 million black holes in our galaxy,” says Michael Muno of the University of California at Los Angeles.
But how can you see something completely black? Although black holes themselves don’t emit light, the region around one can be bright if it is in a binary system with a companion star circling close to it. The hole’s gravity will pull material off the companion to form a swirling disc around the black hole. As it spirals ever inwards, this material becomes so hot it can emit X-rays.
With the advent of orbiting X-ray telescopes in the 1970s, astronomers spotted their first candidate black-hole binary, Cygnus X-1, which lies about 8000 light years away. As well as telltale X-ray emissions, the slight wobble of the system’s bright star reveals that it has a dark companion at least 7 times as massive as the sun. That object is simply too heavy and too dark to be anything but a black hole.
Some hundred other black hole suspects have since turned up in the Milky Way. And in January at a meeting of the American Astronomical Society, Muno’s team reported observations from NASA’s Chandra X-ray observatory that suggest there are tens of thousands of dense, compact objects milling around near the middle of the galaxy, many of them black holes. The likelihood is that they formed farther out: theory predicts that heavy black holes will gradually sink into the galactic centre over billions of years, kicking lightweight stars outward as they go.
Monster munch
AT THE heart of the Milky Way lies a monster. By watching the speeds of stars flying around in the centre, astronomers have shown that it harbours a supermassive black hole – a colossal nugget 3 million times as massive as the sun. It is so immense that its gravity prevents anything, including light, escaping from inside a radius of about 7.7 million kilometres, or about 20 times the distance from Earth to the moon.
Astronomers are finding that most, if not all large galaxies harbour a supermassive black hole. The origins of these holes are unknown. They may pre-date galaxies, forming when giant clouds collapsed in the dark, starless universe. Or they may have formed later inside existing galaxies, when lots of smaller black holes somehow merged.
“I would like to know the final fate of the galaxy. We know that it will collide with the Andromeda spiral, but what will be the product of this encounter?”
Stars, gas and dust swirling towards these giant black holes emit enormous amounts of energy, and power a whole range of ultra-bright “active galaxies”. Yet curiously, our own supermassive black hole looks unusually sleepy and dim, despite having plenty of material to gobble. “In other galaxies, it’s common to find black holes 100 million times brighter,” says Mikhail Revnivtsev of the Space Research Institute in Moscow, Russia, and the Max Planck Institute for Astrophysics in Garching, Germany.
However, earlier this year Revnivtsev and his colleagues announced evidence that the apparent feebleness of our supermassive black hole is just a passing phase. Only 350 years ago, as viewed from Earth, it was a million times brighter than it is now. The evidence comes from a satellite called Integral, which in 2003 revealed extremely energetic X-rays coming from a cloud of hydrogen called Sagittarius B2, about 350 light years to one side of the black hole.
The only plausible explanation is that to observers on Earth some 300 to 400 years ago, around the time that Newton and Galileo were alive, the galactic centre’s black hole would have looked incredibly bright at short wavelengths. So if Newton or Galileo had invented a first-rate gamma-ray telescope, they would have seen a dazzling display from the galactic centre. The bright radiation reached the B2 cloud 350 years after the flare-up, and we now see the cloud fluorescing in X-rays as a result.
The Milky Way’s relatively recent stint as an active galaxy suggests it might well flare up again in future, but there is no telling when.
Megastars
STARS thousands of times wider than the sun, a galactic time bomb and a star so weird that nothing about it makes sense. All these have come to light recently thanks to the latest telescope technology. In January, Philip Massey of the Lowell Observatory in Flagstaff, Arizona, announced that his team had identified the four fattest known stars.
The record-breaking megastars are, in no particular order, KW Sagittarii, V354 Cephei, KY Cygni and Mu Cephei. By measuring the brightness and temperatures of the four stars, the team has estimated their sizes from theoretical models. They calculate that the stars are about 1500 times as wide as the sun. The stars are all about 10 million years old and will probably blow themselves to smithereens in about a million years’ time.
But these four large stars, each with a mass 25 times that of the sun, are by no means the Milky Way’s heaviest. The most massive weigh up to 150 solar masses. Among them is Eta Carinae, a galactic time bomb that weighs at least 120 solar masses. Nuclear reactions inside the star are generating radiation so intense that it exerts an outward pressure similar to the gravity holding the star together.
That makes Eta Carinae violently unstable. Its brightness changes dramatically in just years or decades. Sometimes it is a stellar nobody, invisible to the naked eye – at other times it is one of the brightest stars in the night sky. Astronomers witnessed a giant explosion of Eta Carinae in the middle of the 19th century, but somehow the star lived to tell the tale. It won’t survive much longer though. Eta Carinae will probably blast itself apart within about 100,000 years.
The megastar ranks also contain a few oddballs. A case in point is VY Canis Majoris. If astronomers take VY CMa at face value, it is a humongous 3020 times as wide as the sun. Maybe it has a companion star whose gravity is pulling its atmosphere outwards, but so far no one has found one. Or it is possible that the star has evolved into a strange state never seen before. “Nothing about VY CMa makes sense to me,” says Massey.
Great balls of iron
PONDER this one next time you make a cup of coffee. If you swapped your teaspoon of sugar for a teaspoon of neutron star innards, it would weigh about a billion tonnes. That’s because neutrons stars are amazingly dense, as well as having a host of other exotic properties.
Neutron stars were predicted in theory long before astronomers detected them. Astrophysicists first had the idea back in the 1930s, as they were coming to terms with the notion that when a massive star explodes in a supernova, its core can collapse to form a black hole. They realised that not all supernovae do this, though. If the stellar corpse left behind after the explosion has about 1.3 to 3 times the mass of the sun, it can collapse into a neutron star – a ball of neutrons the size of a small city, with a solid outer crust of iron nuclei.
Neutron stars can also spin as fast as a power drill – some do more than 600 revolutions a second – and have extremely strong magnetic fields. It was these two features working together that first allowed astronomers to detect neutron stars directly.
In 1967, postgraduate student Jocelyn Bell at the University of Cambridge was recording radio signals from the cosmos when she noticed a strange regular “beep” coming from the constellation Vulpecula every 1.3 seconds. She and her supervisor Antony Hewish had no idea what could be causing it. It even crossed their minds that it could be radio broadcasts from “little green men”, so half tongue-in-cheek, they nicknamed the source LGM1.
But it wasn’t long before Bell (now Bell Burnell) ruled that theory out, after discovering similar pulsing signals elsewhere. It was hard to believe that so many communities of green folks across the galaxy were all desperately trying to attract our attention at the same time and all using the same radio frequency. The phenomenon was renamed a “pulsar” and astronomers soon hit on the link with neutron stars.
At a neutron star’s magnetic poles, the magnetic fields energise charged particles until they emit a narrow beam of intense radiation, from X-rays to radio waves. If the Earth is caught in one of these beams, sweeping through space like a lighthouse signal as the neutron star rotates, we can detect its regular flash. This was the pulsing signal that had puzzled Bell and Hewish.
Since 1967, more than 1400 pulsars have turned up in our galaxy. And it’s now possible to detect neutron stars that are not directing polar beams in Earth’s direction, as some emit bright X-rays in all directions when matter from companion stars swirls onto them and becomes extremely hot.
Of all the neutron stars detected so far, about 10 have extraordinarily strong magnetic fields and have been renamed “magnetars”. Their fields are so strong that if one passed halfway between Earth and the moon, it would wipe the data off every swipe card on Earth in one go. Astronomers suspect that magnetars can suddenly release awesome amounts of energy if their magnetic fields suddenly reconfigure. That’s the likely source of a stunning split-second blast of gamma rays that reached Earth on 27 December 2004. The flash was so bright it overwhelmed many satellites’ instruments, even though it is believed to have come from a magnetar some 50,000 light years away.
Speed freak
MOST stars spend their lives circling the galactic centre at a stately pace. So when astronomers announced in February that they had found the fastest star in the galaxy, the discovery raised eyebrows. Not least because it is the first known star that is moving fast enough to eventually escape the Milky Way’s gravitational pull and embark on a lonesome journey into dark intergalactic space.
A team led by Warren Brown at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, stumbled on the high-speed star while trying to pin down the mass of the Milky Way. To this end, they were using a telescope in Arizona to measure the speeds of some stars in the galactic halo when they noticed one was streaking out of the galaxy at about 700 kilometres per second – more than three times as fast as the sun’s speed around the galactic centre. “We have never before seen a star moving fast enough to completely escape the confines of our galaxy,” says Brown.
Astronomers suspect the star has been catapulted out by the supermassive black hole at the centre of the Milky Way. Theory predicts that the black hole could disrupt a binary star system of two stars orbiting each other so that one star goes into orbit around the black hole – or gets sucked in completely – while the other is flung out of the Milky Way. And sure enough, the speedy star is whizzing directly away from the galactic centre.
Hostile takeover
NO GALAXY evolves in total isolation. In fact, galaxies are continuously on the move, and the gravitational attraction between them often puts them on collision course. But the crashes are not necessarily that violent. In a galaxy, stars or multiple star systems are typically 3 light years apart. So when galaxies collide, their stars rarely hit each other, although they do change their orbits. Interstellar clouds within galaxies do collide, though, then collapse under gravity, triggering the formation of new stars.
Rosemary Wyse of Johns Hopkins University in Baltimore, Maryland, studies our own galaxy’s history of mergers. She says the best way to track down alien galaxies inside the Milky Way is to look for stellar misfits: a large group of stars moving in a different direction to its neighbours, for instance, or stars with unusual chemical compositions or unexpected ages. In this way, astronomers have identified one victim that has clearly been gobbled by the Milky Way: the Sagittarius dwarf elliptical galaxy. On the opposite side of the Milky Way disc from us, this alien dwarf galaxy contains some 30 million stars, mostly yellowish old ones, and 15 times that much mass in dark matter. It is moving up through the disc at some 250 kilometres per second.
The structure of the Milky Way’s disc implies a much larger merger in the distant past. Embedded within the Milky Way’s thin disc, which is made of stars of all ages, is a thicker one containing only old stars. Wyse says the likely explanation is that the Milky Way cannibalised another galaxy about a tenth its size, or several smaller ones, at least 10 billion years ago. The gravitational interaction would have puffed out the stars that had already formed in the Milky Way to make the thicker part of the disc. “We can’t say if it was one, two or three mergers, but whatever happened, it was a long time ago,” says Wyse.
But it looks as though the Milky Way won’t gobble too much more in the near future. There are a dozen or so dwarf galaxies orbiting within its halo, but Wyse says they could remain there for billions of years because their gravitational tug is small. Less certain is the fate of our larger galactic neighbours, the Large and Small Magellanic Clouds. The Milky Way is already tugging stars out of them. But Wyse says they may not have time to merge completely with the Milky Way before our galaxy suffers the biggest shock of its life: an encounter with its giant rival, the Andromeda galaxy (see page 40).
“Is there any other technological intelligence out there? Are there any other beings with history, art, music, science, poetry, politics? Or are we the only show in town?”
Mysterious dark halo
MOST of the Milky Way is invisible, which has made it all the more difficult to figure out its structure. The motions of stars appear to be influenced by the gravity of vast amounts of mysterious “dark matter” in a giant ball enclosing the galactic disc and bulge. It is difficult to map this dark halo precisely, but it seems to be several hundred thousand light years wide.
What is it made of? It almost certainly contains some ordinary matter, such as stars too dim to see. But theory suggests that it also includes some kind of exotic matter unknown to science, such as WIMPs (weakly interacting massive particles) that are completely incapable of emitting light.
If there is any dark matter in the solar system, it can’t be very common, or else we would notice its gravitational pull on the planets. But there might still be a faint wind of WIMPs streaming through Earth all the time. Several experiments are trying to detect these particles, but there have been no firm sightings so far.
Alien worlds
FEW discoveries have surprised astronomers as much as the amazing gaggle of alien planets that has turned up over the past decade. “Virtually everything we thought we knew about planetary systems prior to 1995 was completely wrong,” says Paul Butler, a planet-hunter at the Carnegie Institute of Washington.
So far, around 150 planets have been spotted circling stars other than the sun. About 20 of them are in systems of two, three or four planets. Most have revealed themselves when stars have been seen wobbling a little under the gravitational pull of their orbiting planets.
According to Butler, the search has turned up surprise after surprise. It had been expected that most planetary systems in our galaxy would look like the solar system: small rocky planets like Earth closest to the star, gas giants like Jupiter further out, and all following fairly circular orbits. But the vast majority of planets turn out to be in elongated orbits. And gas giants can circle their stars at any distance: many so-called hot Jupiters are closer to their stars than Mercury is to the sun and orbit in as little as three days. Forget the three-day week, this is a three-day year.
The short history of planet-hunting is studded with oddities. The first hot Jupiter was found orbiting the star 51 Pegasi with a period of just four days. Another, orbiting the star HD 209458, also in Pegasus, was the first planet seen crossing the face of its star. By analysing the starlight diffusing though the planet’s fuzzy atmosphere, astronomers have detected the elements hydrogen, oxygen, carbon and sodium in it. But perhaps the most bizarre planetary system yet found is the one orbiting a pulsar 1500 light years away. Its discoverer, Alex Wolszczan of Penn State University, Pennsylvania, says observations suggest at least four rocky planets are orbiting the pulsar, one of them even smaller than Pluto.
But the vast majority of the planets we know about orbit ordinary stars within 200 light years of the solar system. That is simply because nearby planets are easiest to find, and it means that in the Milky Way as a whole there must be an enormous number of planets. The two leading planet-search groups have found planets around 10 per cent of the stars they looked at, and many of the others could have planets that are as yet too hard to see. “I would not be at all surprised if most stars have planets,” says Butler. In that case, think big: there are billions of planets in our galaxy.
Within the next 10 to 20 years, planet-hunters with improved telescopes expect to find small, rocky planets very close to their stars. That will give a clue to how many alien Earths dot the galaxy. Because our solar system is probably not unique we may also find the first genuine solar system lookalikes, with inner rocky planets and outer giants. Further down the line, there are plans for two space-based missions – NASA’s Terrestrial Planet Finder and the European Space Agency’s Darwin, a flotilla of six space observatories – to probe the planets for signs of life.
“My top unsolved puzzle concerns gravitational waves. These ripples in space-time were predicted by Einstein and were observed indirectly by pulsar astronomers, but have not yet been detected directly. When will we see gravitational waves, and will the first detection be from a pair of neutron stars spiralling towards each other and their doom?”
Clash of the titans
OF THE 40 or 50 nearest galaxies, only the Andromeda galaxy matches the Milky Way for size. It is a safe 2.2 million light years away. But with every minute that passes, the gap closes by about 8000 kilometres. The Milky Way and the Andromeda galaxy are racing towards each other at a relative speed of 500,000 kilometres every hour, and in 3 billion years’ time, the two giants will run into each other in a catastrophic encounter that will change them both beyond recognition.
John Dubinski at the University of Toronto, Canada, has simulated this collision on supercomputers (). His simulations suggest that over a billion years or so, the galaxies might swing through each other two or three times, stretching out long wispy streams of stars, before settling into a fairly shapeless elliptical galaxy. “The two galaxies become intertwined, they destroy each other’s structure and the whole thing turns into a big amorphous blob,” says Dubinski.
By this time, the ever-brightening sun will have scorched Earth into a lifeless planet. Maybe humans, or whatever intelligent life succeeds us, will have escaped to an outlying moon such as Titan, which could by then be warm enough for life. Whoever they are, any living inhabitants of the solar system will be treated to a spectacular view of Andromeda approaching and filling the sky, which Dubinski has also simulated.
The final fate of the sun is not clear. It might sail out into intergalactic space on a stellar wisp, or end up in the hectic centre of the merged galaxy, where compressed gas will trigger vigorous star formation, and the night sky would be ablaze with brand new stars and stellar explosions.
What’s certain is that the beautiful spiral order of the Milky Way and the Andromeda galaxy will be lost for good. “If you’re obsessed with visual beauty, I guess that’s a bit sad,” says Dubinski. “But maybe after the collision there will be a different kind of beauty.” He says that although elliptical galaxies look much more boring than grand spirals from the outside, the stars inside them have much more complex, interesting orbits. Or as the old cliché goes – it’s what’s inside that counts.
Obscure beginnings
HOW did the Milky Way form? It’s a question that is stubbornly difficult to answer. The galaxy’s oldest stars are roughly 13 billion years old, suggesting they formed less than a billion years after the universe began life in a giant explosion 13.7 billion years ago. The big bang created a hot, super-dense fireball that gradually expanded and cooled. But this fireball was not completely uniform; rather, it developed myriad dense patches that somehow seeded the clumpy galaxies we see today.
However, astronomers are vague on the details. Did small stars or star clusters form first, then clump together under gravity to form galaxies? Or did gas and dust in the young universe first form huge structures that only later fragmented and collapsed into galaxies that spawned stars?
An ambitious European Space Agency (ESA) mission called Gaia aims to shed light on this. Due for launch in 2010, the Gaia satellite will conduct a census of a billion stars in the Milky Way, gauging their positions, distances, speeds and composition. “I picture it as taking the galaxy apart and seeing how it works,” says David Southwood, ESA’s science director. The mission will allow astronomers to pin down the ages of the stars and their trajectories through the Milky Way with unprecedented precision.
