Stars form from dense clouds of gas and dust called the interstellar medium. Young stars undergo two main phases - the protostar phase where they are still contracting and surrounded by an accretion disk, and the pre-main sequence phase where the star reaches stability but is still gaining mass. Star formation is ongoing in many galaxies and can be observed where interstellar material is concentrated, like nebulae which glow due to ionized hydrogen. Understanding star formation required knowledge of stellar structure and evolution as well as the discovery of the interstellar medium as the raw material for new stars.

1. Astronomy 2
Overview of the Universe
Spring 2007
9. Lectures on Star Formation.

2. Star formation and evolution of
pre-main-sequence stars

3. • 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.

4. 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.

5. 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.

6. 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.

8. Betelgeuse Bellatrix
Rigel
Orion
nebula

9. The Orion nebula with the Trapezium cluster near the
center of the image. This young cluster contains
about 2000 stars.

10. 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

12. 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

14. Interstellar matter is concentrated in the disk of
our galaxy.

15. COBE infrared edge-on image of our galaxy

17. •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.”

18. The Horsehead Nebula viewed
by the AAT

19. Close-up view of the Horsehead Nebula as seen by
HST (left) and AAT (right)

20. 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

21. 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

22. Rosette nebula
The bright stars in the
center have carved out
a cavity in the nebula
through their intense
radiation and winds

23. 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.

24. How do stars form?

25. Process of
star formation
begins with collapse
of unstable cloud of
gas and dust.

26. 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

28. 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

29. -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.

30. Here the star is covered, so that its light does not overpower the
much fainter reflected light from the dust

32. 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.

33. 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.

34. 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.

35. Reflection nebula in
Orion with newly born
star.
Star has T=10,000K and
is about 3 times more
massive than the Sun: a
main sequence A star.

36. Eagle Nebula
-about 20 pc across.

37. “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.