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Discovery triples the number of stars

Side-by-side comparison of stars in the Milky Way and a distant galaxy

Filtering out the light from brighter stars, astronomers detected the faint signature of small, dim red dwarf stars in nearby elliptical galaxies (right), and found these are much more numerous than in our own Milky Way (left). This finding suggests that the total number of stars in the universe could be up to three times higher than previously thought. (Artist's impression)

  • Red dwarf stars seen in distant galaxies for first time
  • There are far more of them than expected
  • Increases the number of potential planets in the cosmos

ASTRONOMERS HAVE DISCOVERED that small, dim stars known as red dwarfs are much more prolific than previously thought—so much so that the total number of stars in the universe is likely three times bigger than realised.

Because red dwarfs are relatively small and dim compared to stars like our Sun, astronomers hadn’t been able to detect them in galaxies other than our own Milky Way and its nearest neighbours before now.

As such, they did not know how much of the total stellar population of the universe is made up of red dwarfs.

Now astronomers have used powerful instruments on the Keck Observatory in Hawaii to detect the faint signature of red dwarfs in eight massive, relatively nearby galaxies called elliptical galaxies, which are located between about 50 million and 300 million light-years away.

They discovered that the red dwarfs, which are only between 10 and 20 percent as massive as the Sun, were much more bountiful than expected.

“No one knew how many of these stars there were,” said Pieter van Dokkum, a Yale University astronomer who led the research, which is described the December 1 advanced online edition of the journal Nature.

“Different theoretical models predicted a wide range of possibilities, so this answers a longstanding question about just how abundant these stars are.”

A galaxy

More red dwarf stars in other galaxies potentially means more habitable planets.

Other galaxies are not like ours

The team discovered that there are about 20 times more red dwarfs in elliptical galaxies than in the Milky Way, said Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics, who was also involved in the research.

“We usually assume other galaxies look like our own. But this suggests other conditions are possible in other galaxies,” Conroy said. “So this discovery could have a major impact on our understanding of galaxy formation and evolution.”

For instance, Conroy said, galaxies might contain less dark matter—a mysterious substance that has mass but cannot be directly observed—than previous measurements of their masses might have indicated. Instead, the abundant red dwarfs could contribute more mass than realised.

In addition to boosting the total number of stars in the universe, the discovery also increases the number of planets orbiting those stars, which in turn elevates the number of planets that might harbour life, van Dokkum said.

In fact, a recently discovered exoplanet that astronomers believe could potentially support life orbits a red dwarf star, called Gliese 581.

“There are possibly trillions of Earths orbiting these stars,” van Dokkum said, adding that the red dwarfs they discovered, which are typically more than 10 billion years old, have been around long enough for complex life to evolve.

“It’s one reason why people are interested in this type of star.”

Adapted from information issued by Yale University / NASA / ESA.

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Giant galaxy hides its secrets

Galaxy NGC 4696

A supermassive black hole beats at the heart of huge elliptical galaxy NGC 4696.

This picture, taken by Hubble’s Advanced Camera for Surveys, shows NGC 4696, the largest galaxy in the Centaurus Cluster (also known as galaxy cluster Abell 3526).

NGC 4696 is an elliptical-shaped galaxy with a difference. Lacking the complex structure and active star formation of their spiral galaxy cousins, elliptical galaxies are usually little more than shapeless, albeit huge, collections of ageing stars.

Most likely formed by collisions between spiral galaxies, elliptical galaxies experience a brief burst of star formation triggered as interstellar dust and gas clouds crash into each other.

But this burst of star formation activity quickly leaves young elliptical galaxies exhausted. With no more gas to form new stars from, the galaxies grow older and fainter.

But NGC 4696 is more interesting than most elliptical galaxies.

A huge dust lane, around 30,000 light-years across, sweeping across the face of the galaxy is one way in which it looks different from most other elliptical galaxies. Viewed at certain wavelengths, strange thin filaments of ionised hydrogen gas are visible within it.

Looking at NGC 4696 at the optical and near-infrared wavelengths seen by Hubble gives a beautiful and dramatic view of the galaxy. But in fact, much of its inner turmoil is still hidden from view in this picture.

At the heart of the galaxy, a supermassive black hole is blowing out jets of matter at nearly the speed of light. When looked at in X-ray wavelengths, such as those visible from NASA’s Chandra X-ray Observatory, huge voids within the galaxy become visible, telltale signs of these jets’ enormous power to “clear out” large volumes of space of their gas and dust.

Adapted from information issued by ESA / Hubble and NASA.

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