Lecture 20 Star Formation

Announcements Comet Lovejoy will be a late night/early morning object through the rest of the semester, so currently there is no plan to have a group observing session. Comet Lovejoy will be a late night/early morning object through the rest of the semester, so currently there is no plan to have a group observing session. This is National Dark Sky Week. Turn off unnecessary outdoor lighting and enjoy darker skies. Also, be sure to check out the web site at This is National Dark Sky Week. Turn off unnecessary outdoor lighting and enjoy darker skies. Also, be sure to check out the web site at

Review We said last time that stars form from giant molecular clouds. We said last time that stars form from giant molecular clouds. But how does a giant, cold cloud of gas become a hot, dense star? But how does a giant, cold cloud of gas become a hot, dense star?

Pressure and Gravity Any gas cloud in space with a temperature above absolute zero has internal pressure. Any gas cloud in space with a temperature above absolute zero has internal pressure. Higher temperature = Higher internal pressure. Higher temperature = Higher internal pressure. A gas cloud’s internal pressure pushes out. A gas cloud’s internal pressure pushes out. Makes the cloud expand. Makes the cloud expand.

Pressure and Gravity Gravity pulls inward. Gravity pulls inward. More mass = more gravity More mass = more gravity –Higher density = more mass –So higher density = more gravity Gravity makes a gas cloud shrink down (collapse). Gravity makes a gas cloud shrink down (collapse).

Stability Pressure > Gravity, cloud expands (gets bigger) Pressure > Gravity, cloud expands (gets bigger) Pressure < Gravity, cloud contracts (gets smaller) Pressure < Gravity, cloud contracts (gets smaller) Pressure = Gravity, cloud stays the same size (hydrostatic equilibrium) Pressure = Gravity, cloud stays the same size (hydrostatic equilibrium)

How Do You Form Stars? Molecular Clouds: Molecular Clouds: –Pressure about equal to gravity –Stable, but “primed” for collapse What could trigger a molecular cloud to collapse? What could trigger a molecular cloud to collapse? –Lower temperature (reduce internal pressure) –Increase density (increase gravity)

How Do You Form Stars? Just need a little “push” to trigger the collapse Just need a little “push” to trigger the collapse What provides the push? What provides the push? –Shockwaves: –Supernovae –Hot winds from new stars

Star Formation Triggers More Star Formation In a molecular cloud, star formation frequently happens in stages. In a molecular cloud, star formation frequently happens in stages. The cloud collapses and fragments. The cloud collapses and fragments. Massive stars form very quickly. Massive stars form very quickly. They heat up the cloud and prevent lower mass stars from forming. They heat up the cloud and prevent lower mass stars from forming. Called an OB association. Called an OB association.

Star Formation Triggers More Star Formation O and B stars don’t live very long. O and B stars don’t live very long. Explode when they die (supernovae). Explode when they die (supernovae). The shock waves spread out into the surrounding cloud, and trigger the formation of smaller, less massive stars. The shock waves spread out into the surrounding cloud, and trigger the formation of smaller, less massive stars.

Star Formation Triggers More Star Formation Also, the hot winds from massive stars can spread out through the surrounding gas clouds, triggering new star formation. Also, the hot winds from massive stars can spread out through the surrounding gas clouds, triggering new star formation.

Let The Collapse Begin! Shockwave increases density of cloud. Shockwave increases density of cloud. Gravity wins! Cloud begins to collapse! Gravity wins! Cloud begins to collapse! –Temperature through cloud not even. Coldest spots collapse fastest. –Causes cloud to “fragment” into collapsing knots. –Each knot is where a new star will form.

Time For Pancakes As a gas cloud collapses, it begins to rotate faster and flatten into a disk (conservation of angular momentum). As a gas cloud collapses, it begins to rotate faster and flatten into a disk (conservation of angular momentum). The knot at the center is surrounded by a flattened disk of rotating gas called an accretion disk. The knot at the center is surrounded by a flattened disk of rotating gas called an accretion disk. The accretion disk: The accretion disk: –Feeds matter onto the protostar –Is where planets form

A Baby Star Begins To Form Matter begins to pile up at the center of the knot. Matter begins to pile up at the center of the knot. –Gravitational collapse releases thermal energy. –Center of knot begins to heat up as collapse continues. –Contracts into a spherical “glob” of hot gas at the center of the knot. Called a protostar.

Daily Quiz 16 – Question 1 What force causes the contraction of a cloud of interstellar matter to form a star? What force causes the contraction of a cloud of interstellar matter to form a star? A.The electrostatic force. B.The strong nuclear force. C.The weak nuclear force. D.The gravitational force.

Protostars As cloud heats up, surface gets hotter. As cloud heats up, surface gets hotter. Protostar begins to generate light. Protostar begins to generate light. Starts out relatively cool, but very large, giving it high Luminosity. Starts out relatively cool, but very large, giving it high Luminosity. Surface temperature increases very slowly as collapse continues, so protostar gets hotter, but fainter. Surface temperature increases very slowly as collapse continues, so protostar gets hotter, but fainter.

Protostars Core of protostar gets hotter for two reasons: Core of protostar gets hotter for two reasons: –Thermal energy from gravitational collapse. –Thermal energy from nuclear fusion. Fusion reactions begin as soon as the center of the protostar gets hot enough (~1 million K) Fusion reactions begin as soon as the center of the protostar gets hot enough (~1 million K) Hydrogen to Helium fusion starts out slowly Hydrogen to Helium fusion starts out slowly Heat doesn’t provide enough outward pressure to stop collapse. Heat doesn’t provide enough outward pressure to stop collapse. As core temperature goes up, fusion reactions happen faster. As core temperature goes up, fusion reactions happen faster. More energy = slower collapse. More energy = slower collapse.

Protostars Near end of collapse, increasing energy output from fusion makes protostar unstable. Ejects lots of material: Near end of collapse, increasing energy output from fusion makes protostar unstable. Ejects lots of material: –T Tauri stars –Herbig-Haro objects Increasing rate of fusion heats up protostar’s surface quickly as it collapses. Gets hotter, but stays the same luminosity. Increasing rate of fusion heats up protostar’s surface quickly as it collapses. Gets hotter, but stays the same luminosity.

Daily Quiz 16 – Question 2 What happens to the temperature and density inside a collapsing protostar? What happens to the temperature and density inside a collapsing protostar? A.Temperature and density both increase. B.Temperature and density both decrease. C.Temperature increases and density decreases. D.Temperature decreases and density increases.

Here’s a Picture of a T- Tauri Star It’s ejecting a huge bubble of material from its outer surface into space.

T-Tauri Stars Exclusively low-mass Exclusively low-mass –All are no more than 3 times as massive as the sun. Very unstable Very unstable Some nuclear fusion, but not enough to support the star (still collapsing) Some nuclear fusion, but not enough to support the star (still collapsing) Likely the sun went through this phase 4.6 billion years ago. Likely the sun went through this phase 4.6 billion years ago. May have a role in clearing out nebula gases from a forming solar system May have a role in clearing out nebula gases from a forming solar system

Here Are Pictures Of Herbig-Haro Objects The protostar is in the center (concealed by dust) These are jets of material coming off of the protostar

Herbig-Haro Objects Caused by the accretion disk getting in the way of mass flowing off of the unstable protostar. Caused by the accretion disk getting in the way of mass flowing off of the unstable protostar. Focuses the hot gasses into the two jets seen in these objects. Focuses the hot gasses into the two jets seen in these objects. Accretion disk Jet

Protostars Collapse stops when heat released by nuclear fusion provides enough pressure to prevent further contraction. Collapse stops when heat released by nuclear fusion provides enough pressure to prevent further contraction. Newly formed star is now stable. All energy to prevent further collapse provided by H to He fusion. Newly formed star is now stable. All energy to prevent further collapse provided by H to He fusion. Star becomes main sequence. Star becomes main sequence.

Hydrostatic Equilibrium Outward pressure force must exactly balance the weight of all layers above everywhere in the star. This condition uniquely determines the interior structure of the star. This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-Russell diagram.

Pressure vs. Gravity

Daily Quiz 16 – Question 3 What eventually halts the slow contraction of a newly forming star? What eventually halts the slow contraction of a newly forming star? A.A second shock wave. B.Electrostatic repulsion. C.Nuclear fusion. D.Gravity.

How Long Does It Take? More massive protostars have more gravity, so collapse is faster. More massive protostars have more gravity, so collapse is faster. Core gets hotter much faster, so stable hydrogen to helium fusion sets in faster. Core gets hotter much faster, so stable hydrogen to helium fusion sets in faster. From initial collapse to main sequence: From initial collapse to main sequence: –Takes only about 30,000 years for a massive star (30 solar mass star). –Takes about 30 million years for a star like the sun. –Takes very long (1 billion years) for a low mass star (0.2 solar masses).

Mass Limits Stars have limits to their mass. Stars have limits to their mass. If the protostar has less than 0.08 solar masses: If the protostar has less than 0.08 solar masses: –Never gets hot enough for stable hydrogen to helium fusion. –Star is “stillborn” –Starts with a surface temperature of about 3,500 K, then cools down. –Called a brown dwarf

Mass Limits If the protostar has more than about 120 solar masses: If the protostar has more than about 120 solar masses: –The core temperature rises very fast. –Star’s internal pressure rises so fast it completely overcomes gravity –Star explodes, shedding large amounts of mass –Enough mass is shed to bring star below 120 solar mass limit

Daily Quiz 16 – Question 4 Why can’t you form a star with less than 0.08 Solar masses? Why can’t you form a star with less than 0.08 Solar masses? A.It will become unstable and explode B.Such a small mass would never collapse C.It never gets hot enough for fusion D.You CAN form a star this size, it is called a blue dwarf

The Pressure- Temperature Thermostat Hotter temperatures: Hotter temperatures: –Atomic nuclei move faster. –They “touch” more frequently. –More fusion reactions each second. –More energy released. –HIGHER PRESSURES PUSH OUTWARD. Cooler temperatures: Cooler temperatures: –Atomic nuclei move more slowly. –Less fusion. –PRESSURE GOES DOWN.

The Pressure- Temperature Thermostat If the fusion reactions in a star slow down: If the fusion reactions in a star slow down: –Pressure < Gravity –Star shrinks –Heats up, increasing fusion rate until Pressure = Gravity. If the fusion reactions in a star speed up: If the fusion reactions in a star speed up: –Pressure > Gravity –Star swells up –Cools down, decreasing fusion rate until Pressure = Gravity. Star tends to maintain the same size and temperature: called the Pressure-Temperature Thermostat. Star tends to maintain the same size and temperature: called the Pressure-Temperature Thermostat.

The Main Sequence More Mass means: More Mass means: –More gravity, so the star weighs more. –The star needs to create more internal pressure to support its weight. What Creates More Internal Pressure? What Creates More Internal Pressure? –More nuclear fusion! –More fusion = more heat and light! So more massive stars are brighter and hotter. So more massive stars are brighter and hotter.

Next Time We will finish discussing stellar structure and start talking about stellar evolution We will finish discussing stellar structure and start talking about stellar evolution Read Unit 61 on stellar evolution Read Unit 61 on stellar evolution