Download Star Formation

Star Formation in the
Interstellar Medium
(ISM)
Star
Formation
Ancient History? Or Still Relevant?
Are stars still being born in large numbers today?
Or has the gas long since been mostly used up?
If so, how do we identify regions of star
formation?
How does it happen?
Analogy: a Forest
Is it still fertile?
To decide that,
ignore the old
mature trees.
Look for
saplings and
shoots.
Identifying Stellar Nurseries
Let’s look for extremely young objects!
O/B stars -- the hottest stars, on the blue end of the main
sequence. (They live only short lives, must be recent
arrivals)
HII Regions – clouds of ionized hydrogen gas. Why?
Because the abundant ultraviolet light from young hot
stars is what makes the gas fluoresce. (Note: dying
stars can do the same thing, as in Planetary Nebulae and
Supernova Remnants. So we must be cautious!)
Example:
‘Rosette’
Nebula
..and more


Dusty, gassy regions – the raw materials
of star formation.
Reflection nebulae – newly formed stars
may not be hot enough to fully ionize the
gas, but can still be bright enough to light
up extensive clouds of gas and dust
Examples: the Trifid, the Pleiades
..and more

Dark Clouds and ‘Globules’ – as the original gas
cloud contracts under gravity, its density
increases and for a time it can appear as a small
dark cloud.

Molecular clouds – in the cool confines of dense
clouds, complex molecules can form and survive.
(In the heat of stars, they would be disrupted.)
So if we can detect them (using specialized radio
telescopes), we deduce the presence of a big
cool cloud of material, ready for gravity to cause
its collapse.
For
example:
…and more

Infrared sources – as a dense cloud
contracts under the influence of gravity,
the forming “proto-stars” steadily warm
up. They will glow vigorously at infrared
wavelengths long before they are hot
enough to ignite thermonuclear reactions
and become actual stars
Orion in Visible Light
and in Infrared Light
…and yet more!


Stars not yet on the main sequence – find a cluster in
which the massive stars are already on the main
sequence, but the low-mass stars are not yet there.
This is because their gravity is weaker, and they collapse
somewhat more slowly as they form!
T Tauri stars – young stars, including some called T
Tauri stars, typically have strongly enhanced `solar
winds’. (This provided the ‘Magic Broom’ that swept
out unused gas when the solar system formed.)
Example: NGC 2264
…and finally

‘Unbound’ systems – we sometimes find
small groups (like quartets) of stars which
are dynamically unstable.
(Like an impossibly incompatible couple, they will
inevitably separate and drift apart! The fact that they
have not yet done so is evidence that the system is
young.)
One Example: the Trapezium
..in the heart
of the Orion
Nebula
Indeed, Orion is An Ideal Location
for Studying Star Formation!
Note that the nebula is about ½ a degree across –
looking as big as the full moon! It can be seen
as a fuzzy patch with the unaided eye.
It is Relatively Nearby, Conspicuous
- and Easily Studied
Exactly How Do Stars Form?
Various Things to Consider
Imagine an extended cloud of gas in space.
1.
2.
3.
What determines whether it collapses or not
under the influence of its own gravity?
Will it collapse to form a single big star, or
break up into smaller lumps?
How long does the process take? What will we
see as this goes on?
‘Cloudy’ Interstellar Space
The Interstellar Medium (ISM) consists of:


a lot of low-density distributed material
(mainly hydrogen and helium); plus
here and there, denser accumulations
(‘clouds’) made of the same material
Not Like Earth’s Clouds!
In our atmosphere,
the visible material
in the clouds is
water droplets;
but the surrounding
atmosphere is
mainly N2 and O2
Interstellar Space is
Very Nearly a Vacuum
The densities are far lower than we can achieve on
Earth. We simply cannot reproduce and test the
physical conditions in our labs.
A Delicate Balance
In a cloud, gravity pulls the atoms together. But their
random motions (the‘heat’of the cloud) provides a
sustaining pressure. (The same effect sustains the
depth of the Earth’s atmosphere: because the air is
warm, the molecules don’t all ‘fall to the ground.’)
Still, if the cloud is massive enough (lots of selfgravity!) or if it is cool enough (reduced sustaining
pressure), an inward collapse can start.
This is Easier for Big Clouds
The density of interstellar gas, even in the clouds, is so
low that only a really huge cloud will collapse to
from stars (unless the material can be made to cool
off dramatically).
Example: some Giant Molecular Clouds (GMCs) contain
enough material to make 100,000 stars or more!
In general, then, we expect stars to form in large
complexes, not as individuals here and there.
For Example: In Orion
(~1500 light years from us)
The ‘Kleinmann-Low’ Nebula is a strong
infrared source: cool gas and dust.
Will a Large Cloud Form
a Single Superstar?
Could an isolated large
cloud collapse to form a
single star 100,000 times
as massive as the Sun?
If not, why not?
(The biggest stars we
see are only ~100 times
the sun’s mass.)
Such ‘Superstars’ Can’t Exist!
There is a limit to how massive any star can be!
Very massive stars have to be fantastically hot (to hold
themselves up against gravity). This means that they will
be abundantly filled with energetic radiation (light), which
in turn provides a pressure that will actually disrupt the
star.
The so-called Eddington limit is about 150 times the mass
of the Sun. No stars more massive than that can exist!
Instead:
Fragmentation  Clusters
As they contract under gravity, the hugely massive clouds
fragment into smaller pieces, forming star-sized lumps!
(The physics of this is well understood.)
As a result, stars form in big clusters (which may, however,
later dissipate). Globular star clusters, for example,
must have formed in this way.
What Then?
As an individual “proto-star’ lump contracts, it slowly
heats up. It glows first in the infrared, and only later
in visible light (as it gets progressively hotter).
On the other hand, such a proto-star is quite big to
begin with. So it can emit quite a lot of red and
infrared light! But it will gradually shrink in size and
eventually settle down as a main-sequence star
(when it’s hot enough to start nuclear reactions in
the core)
Stars Above the Main Sequence
- but moving towards it!
Note that the
lower-mass stars
take longer to
reach the main
sequence.
(Their gravity is
weaker; they
contract more
slowly.)
Hence, As We
Saw Earlier:
NGC 2264: a young cluster in formation.
The Full Cycle
Gas
 forms stars
 which eventually recycle material (the massive stars
explode as supernovae; the lower-mass stars‘puff off’
a planetary nebula much more quietly)
 later, the recycled gas forms yet more new stars…
Two very important points:
(a) later generations of stars will contain a higher fraction of
heavier elements (there is progressive enrichment); and
(b) this cannot last forever! More and more material gets locked
up in remnants (slowly cooling white dwarfs, neutron stars,
black holes,…)