- Количество слайдов: 37
Astronomy 2 Overview of the Universe Spring 2007 9. Lectures on Star Formation.
Star formation and evolution of pre-main-sequence stars
• Stars have been forming throughout most of history of our Universe. Oldest stars almost as old as Universe. – We see ongoing star formation in many other galaxies.
Major achievements that contributed to understanding of star formation: 1 - Understanding of the structure, composition and evolution of stars 2 - Short-lived bright O and B stars prove that star formation going on in our “present age”, now. 3 - Discovery of the interstellar medium: the space between the stars filled with clouds of gas and dust- raw material for making new stars.
Vast space between the stars is not empty: -filled with gas and dust, called the interstellar medium. This is the raw material out of which new stars are born.
Observing stars in formation very difficult task: -Young stars in formation often hidden from view by the dust and gas clouds in which they are born. -Can’t see them in visible light, and have to rely on IR, radio radiation that penetrates dust. Where do we look for stars in formation? -Where interstellar material, gas and dust, is concentrated.
Betelgeuse Orion nebula Bellatrix Rigel
The Orion nebula with the Trapezium cluster near the center of the image. This young cluster contains about 2000 stars.
Viewing objects at different wavelengths gives information about their structure and composition- very powerful diagnostic tool. Here you can see a region of the Orion nebula in optical (left) and in near IR (right). While the optical image reveals the gas and dust structure in the nebula, the IR-image shows the embedded proto-stars
Effects of interstellar dust grains and gas • • Presence of dust not recognized until ~1930. Produces dimming of light and reddening of light. Explained the existence of O stars that were red Amount of reddening related to amount of dimming, so correction for dust dimming could be made
Interstellar matter is concentrated in the disk of our galaxy.
COBE infrared edge-on image of our galaxy
• Dust surrounding stars can reflect the light of the stars and make reflection nebula (nebula=cloud). • Very dense cloud of dust can hide stars behind it and is called “dark nebula. ” • Warm dust ( a few 100 degrees) can be observed glowing in the IR. • Gas produces interstellar absorption lines in stars- not many elements in proper state of ionization to produce absorption lines in the visible part of the spectrum. • Gas near very hot stars (O stars) can be ionized and excited by the stars and made to fluoresce or emit light. Ionized hydrogen is called “H II” and these emission regions are called “H II regions. ”
The Horsehead Nebula viewed by the AAT
Close-up view of the Horsehead Nebula as seen by HST (left) and AAT (right)
Tarantula Nebula in the Large Magellanic Clouds is the brightest star forming region in the Local Group. Hundreds of O-stars are forming there. The young cluster 30 Doradus that is over-exposed in the center of the upper image, viewed here by HST
Young cluster of stars -formed about 2 million years ago -illuminating a cloud of hydrogen gas and dust. -UV radiation from O and B stars ionizing the hydrogen atoms. = H II region. Recombination of electrons and protons leads to the emission of visible light photons, mostly the red Balmer H-alpha transition (3 -2). -places where the dust is dense block off starlight and nebular emission. Eagle Nebula
Rosette nebula The bright stars in the center have carved out a cavity in the nebula through their intense radiation and winds
The dark clouds known as “globules” The largest globule on top is actually 2 separate clouds that overlap along our line of sight. Each cloud is about 1. 4 ly across and they contain material to make more than 15 stars like our own. This region is 5900 ly away, in Centaurus.
How do stars form?
Process of star formation begins with collapse of unstable cloud of gas and dust.
Birth process of a star can be divided into two main distinctive stages: Proto-star-phase: -Proto-stars still in process of attaining star-like structure. -Proto-stars accompanied by strong outflows and jets -Surrounded by accretion disks. Disk adding more mass onto the proto-star. -Proto-star hidden within cocoon of birth cloud- cannot be seen in visible light. For a low mass star this phase lasts about 100, 000 years. Pre-main-sequence phase. -The star’s mass remains largely constant. Stellar object has become visible in optical and near-infrared light. -Accretion is ongoing but at a much lower rate. -After about 5 million years from the start, the disk is mostly gone, and the Jovian planets can be formed. During this phase star can be placed on H-R diagram
At the center, there is a proto-star, and in the disk, planets may form. High mass stars may not likely form disks, because their intense radiation and winds can carve the disks away. 2 -8 times the diameter of our solar system
-Disks are made up of about 99% gas and 1% dust. Dust is sufficient to make all the planets that we have in our solar system. -Disks appear dark, because are viewed against the bright background of Orion Nebula. Reddish glowing object in the middle is a proto-star: Star hasn’t yet reached the main sequence, no nuclear reactions at center, still contracting. These stars are only about 150, 000 years old.
Here the star is covered, so that its light does not overpower the much fainter reflected light from the dust
Standard theory of planet formation: disk first accretes most of its mass onto the central star. Then in cold outer regions of the disk icy bodies grow into a planetary core of several Earth masses. The core then accretes remaining gas from the disk to form a gas giant planet like Jupiter.
Sources of energy during star formation: Gravity: 1) Kelvin-Helmholtz contraction. -stellar object keeps contracting slowly as a result of gravity. -gravitational energy released increases core temperature. Star keeps contracting to replace energy leaking out in form of radiation. 2) Accretion disks also contribute to total luminosity. Gas gradually falls onto star, releases gravitational energy.
Ultimately core becomes hot enough and dense enough to ignite fusion of H into He. The star is born (becomes a certified star) when its luminosity is provided fully by fusion. It then becomes a main sequence star.
Reflection nebula in Orion with newly born star. Star has T=10, 000 K and is about 3 times more massive than the Sun: a main sequence A star.
Eagle Nebula -about 20 pc across.
“Pillars of Creation” -About 0. 3 pc tall and somewhat broader than our solar system. -Lower image a detail of top of the leftmost pillar.