Star Birth

AST 112 Lecture 9

Star Birth

• The Milky Way has 200-400 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!) oF– “Absolute Zero Temperature” is at -459 oF

• 300 molecules per cm3

– Earth’s atmosphere at sea level has 1018 more molecules per cubic centimeter

• “Lumpy”

Star Clusters

• Most stars form in clusters– Clouds have masses of hundreds of Msun

– 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 Nebula Orion 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

stronglyNGC 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 MSun

• Pistol Star (150 MSun)

• 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?