Where do Stars Form?

Transcription

1 Where do Stars Form?

2 Coldest spots in the galaxy: T ~ 10 K Composition: Mainly molecular hydrogen 1% dust EGGs = Evaporating Gaseous Globules ftp://ftp.hq.nasa.gov/pub/pao/pressrel/1995/ txt Slide

3 Star-forming Clouds

4 Star-forming Clouds Interstellar Medium = gas between the stars

5 Horsehead Nebula in Orion

6 Dark Globule, IC 1396

7 Dark Globule, IC 1396

8 NGC4038/4039, Antennae Galaxies

9 Jeans instability: Thermal pressure cannot support the gas cloud against its selfgravity. The cloud collapses and fragments. Slide

10 Collapsing Cloud

11 Collapsing Cloud

12 Collapsing Cloud

13 Formation of Stars Gas clouds have low temperatures, T~ K with densities ranging from n ~ 5 x 10 8 m -3 to >10 10 m -3. Stars form from the gravitational collapse of these clouds. What is the condition for collapse? When does gravity overcome the gas pressure? The Jeans Criterion Worked out by Sir James Jeans ( ), who considered small deviations of a spherical gas cloud from hydrostatic equilibrium. We start with the Virial Theorem: 2K + U = 0 If 2K > U, then the gas pressure will dominate over gravity. If 2K < U, the cloud will collapse. We worked out previously that for a spherical cloud of constant density, U = -(3/5) (GMC 2 ) / RC where MC and RC are the mass and radius of the gas cloud. Also, K = (3/2) N k T where N is the total number of particles, N = MC / (μmh), μ is the mean molecular weight.

14 Formation of Stars The Jeans Criterion U = -(3/5) (GMC 2 ) / RC K = (3/2) N k T N = MC / (μmh) By the Virial theorem, the condition for collapse is 2K < U : 3MC k T μmh < 3 GMC 2 5 RC Radius can be replaced by the initial (constant) density, RC = (3 MC / 4πρ0) 1/3. Substituting into the above and solving for the mass, we get the minimum mass in the cloud for gravitational collapse, this is the Jeans Criterion, MC > MJ. 5 k T MJ = ( ) ( ) 4πρ 0 GμmH 3/2 1/2 3 Jeans Mass Similarly, we can get the minimum radius for collapse, RC > RJ : ( 15 k T ) 1/2 Jeans Length RJ = 4πG μmh ρ0 This is important for Star formation, it is also important for Structure formation after the Big Bang!

15 Shock waves triggering star formation: The Slide Bubble Nebula (Cassiopeia)

16 Stellar Evolution Initial Mass Function (IMF) Shows mass distribution of stars formed Log10 Number per log Mass Nearly 100x as many 0.3 M stars formed than 10 M Current evidence suggests that the IMF is universal, but this has not been tested well. Low mass end and High mass end are still uncertain Log10 Mass [M ]

17 Zero-Age Main Sequence

18 Zero-Age Main Sequence Evolving onto the Main Sequence

19 Protostars: warm clouds of gas surrounded by infalling matter Protostars = pre-birth state of stars: Hydrogen to Helium fusion not yet ignited Still enshrouded in opaque cocoons of dust => barely visible in the optical, but bright in the infrared.

20 A Star is Born

21 Properties of Dust

22 Properties of Dust Infrared Wavelengths μm Dust Obscuration Infrared Radiation UV Wavelengths μm Gas + Dust Dust Grain D ~ 1μm UV/Visible Wavelengths μm Dust Cloud Grain size ~ 1μm Infrared Wavelengths μm Visible Wavelengths μm 1. Dust Absorbs Visible Light, but is Transparent to Infrared Light 2. The Absorbed Light Heats the Dust, Making it Glow in the Infrared

23 Role of angular momentum

24 Protostellar Disks Conservation of angular momentum leads to the formation of protostellar disks birth place of planets and moons (Iω) before =(Iω) after

25 Protostellar Disks and Jets Herbig Haro Objects Disks of matter accreted onto the protostar ( accretion disks ) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig Haro Objects

26 From a protostar to a young star: very hot; still accreting matter Observed in the infrared, because infalling gas and dust obscure light

27 Star emerges from the enshrouding dust cocoon

28 The matter stops falling on the star Nuclear fusion starts in the core Planets can be formed from the remaining disk

29 Life of stars: Gravity is everything Stars are born due to gravitational collapse of gas clouds Star s life is a battle between gas/radiation pressure generated by nuclear reactions and gravity Eventually, a star loses this battle, and gravity overwhelms Slide

30 Slide What happens when all hydrogen is converted into helium in the core??

31 What happens when all hydrogen is converted into helium in the core?? Mass determines the fate of the star Slide

32 Evolution on the Main Sequence A star s life time T ~ energy reservoir / luminosity Energy reservoir ~ M Luminosity L ~ M 3.5 T ~ M/L ~ 1/M 2.5 Slide Massive stars have short lives!

33 Evolution on the Main Sequence Main-Sequence stars live by fusing H to He. Zero-Age Main Sequence (ZAMS) MS evolution Finite supply of H => finite life time. Slide

34 Understanding the Main Sequence

35 Understanding the Main Sequence Luminosity-Radius-Temperature Relation Luminosity = energy/second A star s luminosity is proportional to its size and effective temperature: L =4πR 2 σt 4

36 Mass, the Driving Factor

37 Mass, the Driving Factor Luminosity-Mass Relation L M 4 Main Sequence is also a Mass Sequence Low Mass Stars: cooler & fainter longer MS lifetimes High Mass Stars: hotter & brighter shorter MS lifetimes

38 Star Clusters As gas clouds collapse, they often form stars in clusters ranging from tens of stars to thousands of stars. Every star in a cluster formed from the same cloud, so they have the same metal mass fraction. There are two types. Population II clusters (generically called globular clusters) tend to have older, more metal poor stars. Population I clusters ( galactic or open clusters) tend to be younger, with more metals. Examples: M13 Globular cluster. Thousands of stars, age of 14 billion years. Pleiades Galactic cluster. Many stars, dominated by luminous blue stars, formed within the last 100 Myr.

39 M13 Globular Cluster

40 M13 Globular Cluster

41 M13 Globular Cluster HST IMAGE

42 The Pleiades, Galactic Cluster

43 Slide HR Diagram of a Star Cluster

44 Star Clusters

45 Star Clusters

46 Star Clusters

47 Star Clusters

48 Star Clusters Because Star Clusters were formed all at once, they give us a way of seeing snapshots of stellar evolution. All the stars have (very, very nearly) the same distance modulus, so we only need their apparent magnitudes. Color-magnitude diagram of M3, an old globular cluster. From Renzini & Pecci, 1988, ARAA, 26, 199

49 Star Clusters Color-magnitude diagram for NGC 2362, a very young open cluster. Shows Main sequence and pre-main sequence stars (on left). pre-main sequence main sequence Moitinho et al. 2001, ApJ, 563, L73

50 We can construct theoretical HR (color-magnitude) diagrams for stellar populations as a function of the cluster age. The model for a fixed time is an isochrone. This lets us determine the age of the star cluster and study stellar evolution (are our models correct?!) Age (yrs) at Main Sequence turnoff

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