Stellar evolution Part I of III Star formation

Transcription

1 Stellar evolution Part I of III Star formation

2 The interstellar medium (ISM) The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. Why study the ISM? Dense interstellar clouds are the birth place of stars. Dark clouds alter and absorb the light from stars behind them. Visible only Visible + Infrared Barnard 68 Example: interstellar reddening visible light (especially short wavelengths) is scattered and absorbed by the cloud. However IR radiation is hardly absorbed at all.

3 Nebulae are interstellar cloud of dust, hydrogen, helium and other ionized gas. Nebulae Three types of nebulae: Emission nebula (HII region) a hot star illuminates a gas cloud, excites and/or ionizes the gas, which produces emission lines by de-excitation. Reflection nebula a star illuminates a gas and dust cloud, the light is reflected (scattered) by the dust. Because blue light is scattered at larger angles than red light, the nebula appears blue (Rayleigh scattering cross-section 1/l 4 ) Dark nebula dense clouds of gas and dust that absorb the visible light from the stars behind. Emission nebula Reflection nebula Dark nebula Note: a nebula can be an emission and reflection nebula.

4 Structure of the ISM Two main types of clouds: H I (neutral hydrogen) clouds: Cold (T ~ 100K) clouds of neutral hydrogen (H I) Moderate density (n ~ 10 to a few hundred atoms per cm 3 ) Size: ~ 100 pc Hot intercloud medium: Hot (T ~ a few 1000 K) clouds of ionized hydrogen (H II) Low density (n ~ 0.1 atoms / cm 3 ) Gas remains ionized because of very low density Note: those clouds are stable structure and evolve very slowly.

5 Probing the cold ISM the 21-cm H line Although dust produces most of the obscuration that is readily noticeable, Hydrogen is the dominant component of the ISM (~70% of total mass) with Helium representing most of the rest. Hydrogen found in ISM: H I: neutral hydrogen most of the H in diffuse interstellar medium H II: ionized hydrogen H 2 : molecular hydrogen Unless H I is illuminated by a high-energy source (UV radiation), cold H I cannot be revealed by relying on de-excitation of higher energy states. One (radio) transition is H I can be detected. It is produced by the reversal of spin of the electron relative to the proton. This is a very rare process. The lifetime of the excited state is in the millions of years. But there are many H I atoms!

6 21-cm [1420 MHz] band galactic plane

7 Star formation The birth of stars is still largely an open question details unclear about beginnings of star formation: the protostars. The protostars are formed from interstellar dust and gas clouds collapsing under gravitational attraction But what triggers the original collapse? Recall that the ISM gas clouds are stable (or evolving very slowly). Collapse may be triggered by a shock wave, perhaps produced by a nearby supernova explosion, or perhaps associated with the spiral arms of our Galaxy.

8 Compressed gas clouds and star formation

9 Shocks triggering star formation Origin of the shock waves? Supernovae, novae or other stellar explosions Stellar winds from hot stars

10 Protostar formation Protostar: a contracting mass of gas that represents an early stage in the formation of a star before nucleosynthesis (nuclear fusion) has begun. Protostars shines but their energy source is NOT nuclear fusion, but the conversion of gravitational potential energy into heat and light (Helmholtz-Kelvin contraction) when the gas / dust collapses and compresses. Using a simplified* model, it is possible to estimate the critical size / mass of a gas cloud given its temperature T and density r 0 over which the gas cloud will collapse. The critical mass is called the Jeans mass and the radius (assuming a spherical cloud) the Jeans length. Jeans mass:! " 5%& '() *, Jeans length: 4 " 15%& 40'() * *See derivation

11 What is µ? Jeans mass:! " 5%& '() *, µ is the mean molecular weight is the average mass of a free particle in the gas in units of the mass of the hydrogen atom. This is a weighting factor that takes into account the star composition (and ionization state) 4 = 6) ) * Using X, Y and Z, the mass fractions of H, He and others ( metals ) respectively, one get*: 1 Neutral: : < + 1 > = 8 Fully ionized: 1 2: + 3 4? 4 < A > =? *See derivation

12 Exercise Determine µ n and µ i for a typical younger star (X=0.70, Y=0.28, Z=0.02)

13 A simple collapse model Very simplified model (no rotation, no magnetic field effect, no outward pressure ) Collapsing gas cloud Protostar formation Collapsing gas cloud of initial density r 0 is free falling at constant temperature (true when the density is low, clearly incorrect when the density increases significantly) Free fall timescale:! "" = 3% 32 1 () *, - Note that the timescale doesn t depend on the initial size and temperature of the gas cloud. Assuming uniform density, all parts of the cloud will take the same amount of time to collapse.

14 Not so simple! Of course, this is not so simple! Two examples: Conservation of angular momentum as the cloud contracts, its core starts spinning faster, so rotation plays an important role in the creation of a protostar. Also outflow jets forming transient Herbig Haro objects. A gas cloud will contract into clumps, which will become the site of multiple star formation. There are limits to how big a single star can be. Time

15 Exercise Calculate the Jean s mass M J for the following two clouds: 1. A typical diffuse hydrogen cloud assumed to be entirely composed of H I (data: T = 50K, r 0 = m H n H = 8.4x10-19 kg.m -3 using n H = 5x10 8 m -3, µ=1). 2. The dense core of a giant molecular cloud (data: T = 10K, r 0 = 2m H n H2 = 3x10-17 kg.m -3 using n H2 = m -3, µ=2). For (2), Deduce the timescale t FF of free-fall collapse. Note: the figure shows the evolution of the collapse following the differential equation derived in class.

16 Giant molecular gas cloud Carbon Monoxide (CO) map

17 Evolution of a protostar in the H-R diagram Eventually, temperatures at protostar core reach 10 million (10 7 ) K. Sufficient to start fusion. At this stage, the protostar becomes a star and it joins the Main Sequence (MS). Before this, the protostar position on H-R diagram depends on its mass Therefore, the final position of the star on the MS also depends on mass

18 Evolutionary tracks Figure shows tracks of protostars of various masses. As protostar collapses, it heats up and moves on the H R diagram. Behavior can be complex! Birth line: the protostar emerges from his dust cocoon. Ignition of thermonuclear fusion. Note: protostars are not red giants, even though they begin their lives in that area of the HR diagram. The normal HR diagrams show positions of stable stars only, not protostars

19 Evolutionary tracks Once the protostar reaches the Main Sequence, it becomes a stable star. The star s core has reached 10 7 K and hydrogen fusion starts The outward thermal pressure balances the inward gravitational pressure and prevents the star from contracting. From that point on, the star remains on the MS quietly burning its reserves of Hydrogen. The heavier (and hotter) the star, the shorter the time it takes for the protostar to become a star. Weaker gravity = slow contraction Weaker gravity / lower mass = small core and higher compression needed to ignite fusion.

20 Evidence of star formation T Tauri stars 10 5 to 10 7 yr old Still in the forming stage. Nebula around S Monocerotis contains many massive, very young stars, including T Tauri stars, which are highly variables and bright in IR.

21 Globules EGG: Evaporating Gaseous Globules ( EGG ): newly forming stars exposed by the ionizing radiations from massive stars. Bok globules: ~10 to 1000 solar masses Contracting to form protostars