What
did the first stars look like?
You
get no credit if you only said big and round! That much is a given.
The
first stars were indeed very big, perhaps over 100 times more massive
than our sun. They were also composed solely of the hydrogen and helium
created in the big bang. These two features---very massive and pure---make
them different from all the stars in existence today. The first stars
were so big and hot that they burned blue, not yellow like our sun.
Let's
start from the beginning---I mean, really the beginning. Scientists say
that space, time and matter as we know it were created 13.7 billion years
ago in a big bang. Another 200 million years would pass before stars formed.
During much of this time, between the fireball of the big bang and the
era of first starlight, the universe was steeped in darkness.
Star
formation occurred when hydrogen and helium created in the big bang slowly
condensed into gas clouds under the force of gravity. There were no other
elements in the universe at this time aside from a tiny amount of lithium.
That is, no oxygen, carbon or the other elements that make life possible.
When
the cloud grew dense enough, high pressure and temperature initiated nuclear
fusion in the cloud core, the signature of star birth.
Stars
glow by first burning hydrogen. The "ash" of all that hydrogen
burning produces helium. Once the hydrogen is gone, the star starts burning
the helium. The "ash" of helium burning is carbon and oxygen.
You might see where this is going. As stars burn, they create new (heavier)
elements from lighter ones.
When
a star runs out of fuel, it explodes. The explosion sends all the newly
minted elements into space. This means that the stars that came after
the first generation of stars were "dirty," containing the dozens
of chemical elements that were floating in space instead of just hydrogen
and helium.
Scientists
have not yet seen these first stars. Being so hot, the stars burned quickly.
They were gone in just a few millions years, as opposed to our sun, which
is about five billion years old.
There
is a debate about how big a star can get. Some scientists say that when
stars have only hydrogen and helium, then can get very big. Most scientists
say the first stars were 50 to 200 times as massive as the sun, yet some
speculate that 1,000-solar-mass stars existed. The presence of heavier
elements forces modern stars to be smaller, no larger than 50 solar masses,
according to theory.
Clearly
there are many unanswered questions!
In
October, scientists at NASA Goddard using the Spitzer telescope said they
might have detected a diffuse glow of the very first stars. They described
the observation as seeing the glow of a distant city at night from an
airplane. The light is too distant and feeble to resolve individual objects.
To actually see the first stars, they said, they will need to wait for
the successor to the Hubble Space Telescope, a mission called the James
Webb Space Telescope.
In
September scientists using the Swift telescope detected the most distant
explosion ever seen, a gamma-ray burst from a very early star that exploded
nearly 13 billion years ago. While this wasn't part of the first generation
of stars, it was sure close. The observation implies that Swift could
detect the first stars when they explode.
These
telescopes are like time machines. The firsts stars exploded 13.5 billion
years ago, but because their light takes so long to reach us, we can still
see what they looked like and even watch them explode.
This
week's question comes from Christopher Wanjek. Mr. Wanjek is a science
writer supporting the Beyond Einstein initiative, a roadmap to understand
the forces of nature beyond General Relativity and Quantum Mechanics through
the study of the Universe from the Big Bang to black holes.
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