Star Birth AST 112 Lecture 9

Star Birth The Milky Way has billion stars. 2-3 stars born per year in our galaxy!

What is this?

Star Birth Star birth occurs in clouds of gas and dust that tend to be: – Cold Molecules are slow, don’t resist collapse – Dense Lots of stuff to make stars out of Called molecular clouds

Cold, Dark Clouds How do we know the dark spot isn’t just devoid of stars?

Cold, Dark Clouds Look at the edges. Stars are red and gradually fade away.

Cold, Dark Clouds Dust doesn’t scatter infrared as effectively as visible light See stars behind cloud if we look in infrared

Star Birth

Star Forming Clouds -450 (that’s negative!) o F – “Absolute Zero Temperature” is at -459 o F 300 molecules per cm 3 – Earth’s atmosphere at sea level has more molecules per cubic centimeter “Lumpy”

Star Clusters Most stars form in clusters – Clouds have masses of hundreds of M sun – Form open clusters of 100+ stars Rarely, but sometimes: – Very small, dense clouds collapse to form one or a few stars We’ll assume a cluster is forming

Cloud Collapse Gravity pulls material inward As the cloud collapses, it gets warmer Causes pressure increase, resists collapse. BUT: Warm molecules release photons, cooling the cloud – This happens so long as the cloud isn’t too dense – Pressure can’t resist collapse yet due to cooling Collapse continues

Cloud Fragmentation The cloud fragments – At the Jeans Mass, gravitational collapse happens very quickly – Dense areas of the cloud reach the Jeans mass first These fragments eventually form protostars

A Fragment Forms a Protostar Eventually, can’t radiate heat because too dense – Too many molecules to run into Eventually, almost all of the radiation is trapped Heat goes up, pressure goes up – contraction slows It is now a protostar. – Bright but no nuclear fusion – They are still embedded in the molecular cloud – How can we hope to observe them?

Protostars emit heavily in infrared, which travels through dust.

Protostar to Main Sequence It moves to the Main Sequence of the HR diagram – Spends most of its life here – Millions to billions of years The O / B stars can actually die before the M stars turn on!

Why so blue? Open clusters often look very blue. Why?

…red stars? This cluster is 50 million years old Red stars turn on in about 150 million years Why are they there?

Back to the Cloud Credit: ESO As the cloud collapses and stars turn on, it begins to glow.

The Eagle Nebula A site of active star formation The entire cloud glows from star formation – Hydrogen glowing red Young, bright blue stars are visible Credit: ESO

Pillars of Creation Stars outside of these columns are “boiling” gas off the top

Pillars of Creation EGGs (Evaporating Gaseous Globules) These are protostars that get uncovered as surroundings boil away

Trifid NebulaOrion Nebula

Trifid Nebula (up close)

Cloud to Cluster Stars slowly clear out surroundings Gas cools Starts to look less like chaotic cloud, more like organized cluster Credit: NASA

Young Star Cluster Young stars still surrounded by dust and gas We see a reflection nebula – Blue light scattered more strongly NGC 346 (In Small Magellanic Cloud)

Open Cluster Later on, just looks like a group of stars Eventually disperse, “mix in” with the galaxy

Newborn Star Size Largest size? Usually top out at 100 M Sun Pistol Star (150 M Sun ) Excessive radiation pressure drives outer mass away

Newborn Star Size Smallest size? 0.08 solar masses Anything less can’t squish hard enough YOU FAIL. GO HOME. Less than 0.08 solar masses: Brown Dwarf

Brown Dwarfs Brown dwarfs are actually either deep red or infrared Try to catch them when they form because they cool off – Best to look in star forming regions

Newborn Star Size Typical size?