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Number of stars in universe may be vastly larger than thought

A new census indicates that there may three times as many stars as current estimates, on account of undercounting red dwarf stars, which are too small and dim to see at great distances.

December 01, 2010|By Amina Khan, Los Angeles Times

Here's a finding that may make some stargazers do a double-take through their telescopes: There may be three times as many stars in the universe as we thought. Fixing this astronomical miscalculation may force some researchers to reconsider what far-off galaxies really look like and how the stars within them came to be.

"It has terrifying implications for a lot of the astronomy we do," said Caltech astronomer Richard Ellis, who was not involved in the work.

Previous star counts relied on the assumption that the larger universe looks much like our galaxy. But authors of a report published Wednesday in the journal Nature say that there are many more red dwarfs — small, dim stars that can't be picked out individually when very far off — in certain other galaxies than in the Milky Way.

The new census, based on analysis of the light signature of the galaxies using instruments at the Keck Observatory in Hawaii, pushes the total number of stars in the universe to 300 sextillion (that's 100 billion squared, multiplied by 30).

Red dwarfs are often a mere 10% to 20% of the mass of the sun, and hundreds of times dimmer. That makes them so faint as to be undetectable at great distances. To make up for what they couldn't see, astronomers for decades have assumed that the proportion of red dwarfs in other galaxies would be similar to the ratio known to exist in our own, said Yale astronomer Pieter van Dokkum, the study's lead author.

Armed with recent advances that enabled them to better detect the faint signals of dim, low-mass stars, Van Dokkum and co-worker Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics looked at the radiation emanating from eight elliptical galaxies between 50 million and 300 million light-years from Earth. These types of galaxies have a bulging shape and typically contain relatively old stars — very different from spiral galaxies like the Milky Way, which have stars arrayed in flat, rotating disks and armlike projections where new stars are developing.

If any galaxy would look different from ours on the inside, it would be these elliptical types, Van Dokkum and Conroy figured.

The scientists looked at the light given off by these galaxies to determine what chemicals were present, and in what amounts — revealing what kinds of stars they came from. They found strong signs of sodium and iron, which are typically found in feeble, low-mass stars. They calculated that the strength of the sodium and iron signatures was enough to raise the red-dwarf estimate within these galaxies by a factor of nine.

If that calculation holds for all elliptical galaxies — which constitute one-third of all known galaxies — that would triple the star census of the universe.

Ellis said the results are not ironclad, in part because the paper relies on an assumption of its own: that red dwarfs in other galaxies have the same proportion of sodium, iron and other chemicals as Milky Way red dwarfs

Should future studies confirm the findings, astronomers may have to redo all kinds of basic numbers.

Among them: The young universe may have had different proportions of stars than it does today. Stars and galaxies may develop differently than scientists believed. There may be slightly less dark matter in the centers of these galaxies than had been predicted.

And with so many more stars available, the probability that an Earth-like planet — and other forms of life — exist in distant parts of space is much higher, too.

"It's a bit like going to another country and realizing the rules and customs you're used to in your own country no longer apply," Ellis said.

amina.khan@latimes.com

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