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Short Sharp Science: A New Scientist blog

Existence: Bright lights of our galaxy's downtown

Roger Highfield, New Scientist magazine editor

Read more: "Existence special: Cosmic mysteries, human questions"

apspace.jpg(Image: NASA/ESA/SSC/CXC/STScl)


The Milky Way is a disc some 100,000 light years across that consists of at least 250 billion stars, possibly as many as a trillion. This composite image, taken by three orbiting observatories - NASA's Hubble, Spitzer and Chandra telescopes - reveals our galaxy's mysterious core. At 26,000 light years away, it is a slice of sky that covers about the same angular width as the full moon.

Yellow represents the near-infrared observations of Hubble, highlighting stars and stellar nurseries. Red provides the infrared gaze of Spitzer, where stellar radiation and cosmic winds make dust clouds glow. The blues and violets show Chandra's observations of X-rays.

X-rays are emitted by gases heated to millions of degrees by stellar explosions. The bright blue blob on the far left reveals emissions from an X-ray binary; a system in which an ordinary star is being stripped of material by a paired neutron star or black hole. X-rays are also emitted when gas swirls and flares around the supermassive black hole which lurks at the centre of the galaxy, which lies within the bright white region to the far right of the picture, just under halfway up.

Its resident black hole, named Sagittarius A*, is more than 4 million times as massive as our sun and is surrounded by a star cluster which makes our neighbourhood seem desolate, says Mark Lovell at Durham University in the UK.

Draw a sphere around the sun as far as Proxima Centauri, the nearest star, and it would include just the sun, some rocks (planets, asteroids and suchlike) and interstellar gas. "Were you to place the same sphere in the white region, however, it would contain enough stellar-mass black holes, stars (surprisingly many of which are young and huge) and other objects to make one million suns, which is nevertheless only a quarter of the mass of Sagittarius A* itself," says Lovell.

Relative to those observed in similar galaxies, our neighbourhood black hole is quiet. In the early universe, though, it could well have been bright enough to outshine the entire galaxy, says Lovell. The galactic core could flare up again, albeit not any time soon.

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Have astronomers ever seen a pair of neutron stars or black holes orbiting each other and sucking gas and dust from a third star?
The extra motion might lead to interesting effects.


The often quoted truism of "a trillion stars" in the Milky Way is probably not correct. Visible stars still number about 100 billion in our galaxy, based on the light we see from it. Observed Brown dwarf transits seem to indicate the "stars" we don't see are about as common as the ones we do see, so the total is probably about 200 billion. The trillion figure comes from meaningless extrapolation of the Initial Mass Function into very low masses, but all the evidence is currently against that being valid.

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