Pretty Pictures!
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Evidence for star formation:
GIANT MOLECULAR CLOUDS (GMCs) -- These are clouds that contain thousands of solar masses of gas and dust and are hundreds of light-years in diameter. Deep inside, these clouds are dark and very cold, between 10 K to 100 K, which allows large molecules to form (some of the proteins found in amino acids have been observed!) These clouds are stable against collapse unless they are disturbed, or "pushed", which can induce gravity to start the collapse of parts of these GMCs.
When this occurs, a large portion of a cloud will start to collapse, but this will not form a single large star. As part of this cloud collapses, it will break up into smaller cloudlets by a process called FRAGMENTATION. The fragmentation continues until the cloudlets have masses in the range of observed stars (0.1 to 100 solar masses). These final fragments are called PROTO-STARS.
The Orion nebula is nearby star forming region with bright glowing gas heated by hot O-stars. While it is impossible to see into these star-forming regions in optical light (ROYGBIV), it is possible to see inside using INFRA-RED CAMERAS.
This is now our "picture" of star formation:
The proto-stars will continue to contract due to gravity. This compression will heat up the core. Eventually, the core becomes very hot and some of this heat will reach the outer layers and heat the surface. During this process, all the energy comes from the gravitational contraction. At some point, the surface temperature becomes high enough so that these proto-stars can be placed in an H-R diagram.
Proto-stars will enter the H-R diagram from the upper right side since proto-stars are large in diameter and have a surface that is slowly becoming hotter. Unfortunately, it is very difficult to observe proto-stars in detail as they are always surrounded by large amounts of gas and dust.
EVOLUTIONARY TRACKS are drawn in an H-R to show how the luminosity (energy output) and surface temperature of a star change during the life of a star. The Evolutionary Tracks for different mass proto-stars are shown here:
The mass of the proto-star will determine two properties of its Evolutionary Track:
Examples of Pre-Main Sequence stars are T Tauri stars which lie just above the MS in an H-R diagram.
The proto-star stage ends when the star reaches the Main Sequence. At this point, the core of the proto-star has become hot enough for nuclear fusion of hydrogen into helium to start. This halts the collapse process as the energy released is able counter-balance the pull of gravity.