What is a star?
A star is a massive ball of gas bound together so tightly by gravity that it heats up to enormous temperatures – tens of millions of degrees Celsius at its center – in order to support itself against further collapse. At such high temperatures, the atoms in these stars undergo nuclear fusion, in which light elements (like hydrogen) collide and stick together as heavier elements. In our Sun, fusion transforms enormous quantities of hydrogen into helium each second. During each of those reactions, a tiny bit of matter is converted into energy, providing the extraordinary luminosity of the Sun. The properties of a star are mostly driven by its mass: more massive stars have more extreme gravity - and hence higher temperatures and luminosities. In the Milky Way, there is a wide range of stellar masses: our Sun is about average, but there are stars ten times smaller and a few stars more than one hundred times more massive!
Image: Full disk view of the sun from SDO, telescope AIA 335 on June 2, 2010.
Credit: NASA Goddard
How did the first star forming clouds form?
Star formation begins with a cloud of relatively dense gas. At the end of the cosmic Dark Ages, the gravity of dark matter clumps grew strong enough to pull normal hydrogen into them. Inside these clumps, gas clouds began to form. Unfortunately, these clouds were not nearly dense enough to form stars, because they were also too hot. Over time, the hydrogen atoms inside them collided and formed molecular hydrogen (two hydrogen atoms stuck together), which themselves would also collide. After these latter collisions, the hydrogen molecules lost some of the collision energy as light – which escaped the gas cloud, cooling it off and allowing it to contract. After millions of years, this slow process allowed enough matter to accumulate at the center to form the first stars in the Universe.
Image: An artist's impression of early star formation.
Artwork Credit: Adolf Schaller for STScI,
Science Credit: NASA and K. Lanzetta
What were the first stars like?
While the Sun is a very typical star in the Milky Way galaxy, the first stars to form were likely very different – while their detailed properties are as yet unknown, their formation process suggests that they were likely tens or even hundreds of times more massive than the Sun. Because of their extreme sizes, these stars had prodigious luminosities, and they consumed their fuel supplies in just a few million years. (The Sun, even though it is so much less massive than these stars, consumes its fuel far more slowly – it will take about ten billion years before it runs out of fuel.) These first stars are known as Population III stars to astronomers.
Image: A depiction of the first stars created with supercomputers.
Credit: Tom Abel and Ralf Kähler
How did the first stars end their lives?
Because they are so massive and compact, the material inside stars feels strong gravity. The only way to support itself is for the material to be extremely hot, so that its intense pressure can resist gravity. But once a star runs out of fuel for nuclear fusion, it has no way to maintain a high temperature – and gravity wins! For the very first, very massive stars, the end of the fuel supply leads to either a supernova explosion, in which nearly the entire star explodes with enormous energies, or the formation of a black hole – a region of such intense gravity that nothing, not even light, can escape.
Image: The Crab Nebula is a modern example of a supernova remnant, a type of dead star.
Credit: NASA, ESA, J. Hester, A. Loll (ASU)
