Presentation on theme: "Inter Stellar Matter with a high enough density, and a low enough temperature for proto-stars to form. Protostars form in cold dark nebulae Star Formation."— Presentation transcript:
Inter Stellar Matter with a high enough density, and a low enough temperature for proto-stars to form. Protostars form in cold dark nebulae Star Formation
Giant Molecular cloud (GMC) in Orion About 1000 GMCs are known in our galaxy These clouds lie in the spiral arms of the galaxy, where the dust & gases are.
Size of cloud – large,Compression area - small Warmer GMCs resist forming stars, kinetic energy opposes the force of gravity to collapse the gas. A cooler gas is needed, and the GMC must be disturbed to induce it to collapse. Size: r ~ 50 pc Mass: > 100,000 M sun Temp.: a few 0 K Interstellar clouds of mostly molecular hydrogen H
Star formation is triggered when a sufficiently massive pocket of gas is squeezed by some external event, such as a shock wave Sources of Shock Waves : (1). Since massive stars die young, Supernovae explosions happen near sites of recent star birth. (2) Previous star formation can trigger further star formation. (Stellar winds) (3) Spiral arms rotating can cause shock waves.
As a proto-star evolves, it shrinks, its density increases and it temperature rises. Proto-stars are physically larger than the main- sequence stars that they will become. What types of stars are formed? OB – Few AFG – More KM – Many, Many When you compress a gas it heats up. When a gas expands it cools
Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars Observations of star formation:
T Tauri Stars Below is a T Tauri star with an accretion disk, and a jet of hot gas. All proto-stars will eject gas before they reach main sequence but the cooler stars G,K, and M, do so more vigorously and are called T Tauri Stars..
Some young disks & jets revealed Low-mass stars that eject gas before becoming main sequence stars, may lose as much as 40% of its mass.
When core temperature ~ 10 Million K: Ignite core P-P chain fusion Stellar wind blows away the cocoon Settles slowly onto the Main Sequence Some of the clump material settles into a rotating disk, from which planets might form. Collapse is slower for lower masses: 1 M sun takes ~30 Myr 0.2 M sun takes ~1 Billion years Low-Mass Proto-stars
Actual Protoplanetary Disks Four protoplanetary disks in the Orion Nebula, 1500 light years away. The disks are 99% gas and 1% dust. The dust shows as a dark silhouette against the glowing gas of the nebula.
Finally: Pressure=Gravity & collapse stops. Star reaches the Zero-Age Main Sequence (ZAMS). To Reach Main Sequence As the core heats up, H fusion runs faster: Core temperature rises Core pressure rises Collapse begins to slow down If the core temperature reaches at least 10 million deg K, the proto-Star becomes a Star
Meanwhile, back in the GMC, things are still happening
Star Form in Clusters Stars do not form isolated, but in large groups, called Open Star Clusters. Our own Sun is part of an open cluster than includes other nearby stars such as Alpha Centauri and Barnard's star. Gravitational interactions between the stars and other objects will cause these clusters to eventually disperse over time
In 1905, Danish astronomer Einar Hertzsprung, and independently American astronomer Henry Norris Russell, noticed that the luminosity of stars decreased from spectral type O to M. To bring some order into the different types of stars: they organize them in a diagram, the H-R diagram
Each star is represented by a dot.The position of each dot on the diagram corresponds to the star's luminosity and its temperature The vertical position represents the star's luminosity. The horizontal position represents the star's surface temperature. H-R Diagram Basics
H–R Diagram or L-T Diagram Notice that the plot is not completely random, so there is some sort of relationship.
H–R Diagram White Dwarfs Giants Supergiants Main Sequence
Cool BRIGHT Color Version of H-R Diagram Stars get hotter Stars get larger
Class Ia,b : Supergiant Class II: Bright giant Class III: Giant Class IV: Sub-giant Class V:MS The Sun is a G2 V star Luminosity classes
R 20 R M 30 M R 5 R M 7 M R 1 R M 1 M (sun!) R 0.3 R M 0.2 M There is a unique mass & radius for each star along the main sequence Radii on the Main SequenceL = 4πR 2 σT 4 If you know L & T, you can calculate R
In the last few years, two new groups were added to the OBAFGKM classification, they are L & T. These stars have been found due to greatly improved infrared detectors aboard satellites. Both L & T are Brown Dwarfs. They are visible in the red, and infrared regions,
Classification of Stars IaBright supergiant IbSupergiant IIBright giant IIIGiant IV Subgiant VMain sequence star
Classification:Spectral Class Alternate way of describing temperature: SPECTRAL CLASS O = 40,000 K B = 20,000 K A = 10,000 K F = 7500 K G = 5500 K K = 4500 K M = 3000 K The spectral classes OBAFGKM began as a method of classifying stars according to the appearance of the absorption lines in their spectra.
900,000 main sequence stars 95,000 white dwarfs 4000 giants 1 supergiant Random Sample of Stars If you took a random sample of 1,000,000 stars from our galaxy. In this sample, you will find, on the average:
Pre-main sequence evolutionary tracks Most everything about a star's life depends on its (MASS).
Life Tracks for Different Masses Higher- mass stars form faster Lower-mass stars form more slowly
Temperature Increases Luminosity Stars more massive than 150M Sun would blow apart **** Stars less massive than 0.08M Sun can’t sustain fusion
Main sequence Zero-age main sequence (ZAMS): ZAMS, phase at which star first gets all its energy from H burning (star no longer contracts). Main sequence (MS): phase of core hydrogen burning, this is the longest stage in stellar life. A star spends 90% of their life on the MS
Main Sequence Lifetimes (predicted) Mass (suns) Surface temp (K) Luminosity (suns) Lifetime (years) 2535,00080,0003 million 1530,00010,00015 million 311,00060500 million 1.57,00053 billion 1.06,000110 billion 0.755,0000.515 billion 0.504,0000.03200 billion
Normal gas Pressure is the force exerted by atoms in a gas Temperature is how fast atoms in a gas move Hotter atoms move faster higher pressure Cooler atoms move slower lower pressure Pressure balances gravity, keeps stars from collapsing
Core-Envelope Structure Outer layers press down on the inner layers. The deeper you go, the greater the pressure. Gas Law : Greater pressure = hotter, denser gas Where fusion takes place Supplies gravity to the core The star develops a Core-Envelope structure: A hot, dense, compact central CORE surrounded by a cooler, lower density, extended ENVELOPE
When there is a balance between the two, we have a condition of Hydrostatic Equilibrium. In this condition, the star neither expands, nor contracts. Stars on the Main Sequence, are in equilibrium. Gravity pulling inward wants to contract the star. Pressure pushing outwards from fusion wants to make the star expand.
Thermodynamics says : Heat always flows from hotter regions into cooler regions. In a star, heat flows from the hot core, out through the cooler envelope, to the surface where it is radiated away as light Radiation Energy is carried by photons. which leave the core, hit atoms or electrons and get scattered. They slowly stagger to the surface. Takes ~1 Million years for a photon to reach the surface.
Convection Energy carried from hotter regions to cooler regions above by bulk buoyant motions of the gas. Everyday examples of convection are boiling water.
Main-Sequence Stars and Fusion 1.Proton-Proton Chain : Low mass stars Relies on proton-proton reactions Efficient at low core Temperatures (T C <18M K) Energy is generated by fusion of 4 1 H into 1 4 He. There are two nuclear reaction paths by which a star might accomplish this fusion: 4 x 1 H 1 x 4He + energy. Fuse 4 protons ( 1 H) into 1 4 He nucleus. This reaction produces the following by-products: Gamma-ray photons, 2 positrons, and 2 neutrinos that leave the Sun.
2. CNO Cycle: High mass stars Efficient at high core Temperatures(T C >18MK ) In stars that are hotter than 18 million degrees Kelvin, protons are fused into 1 Helium nucleus via a multi-step nuclear reaction, where Carbon is the catalyst.
Main Sequence Lifetimes Spectral TypeMass (Solar masses) Main sequence lifetime (million years) O5401 B01610 A03.3500 F01.7 2700 2.7 BY G01.1 9000 9 BY K00.8 14 000 14 BY M00.4 200 000 200BY More massive star will have the shorter life time O & B burn fuel like a bus! M burn fuel like a compact car! Every M dwarf ever created is still on the main sequence!!
Largest Star known: LBV 1806-20 Pistol Star 150-200 solar mass Temp 12,300 K Discovered 1995 Radius 500 time sun’s Distance 45,000 ly Coolest White Dwarf SDDSS-J1403 Mass 0.6 solar mass Temperature 4,300 K WD Radius 0.01 times Sun Distance 145 ly Hottest Star White Dwarf Central star of NGC 2440 Temperature 211,000 K Mass 0.6 solar mass Radius 0.028 times Sun Distance 7,100 ly This star will eject gases into space, and by the time it becomes a main-sequence star, its mass may be 10 solar masses.
Line of Sight Doppler Motion (Radial Motion) Proper Motion (Tangential Motion) (v t ). (v r ) v Actual Motion
Radial Velocity The radial velocity of a star is how fast it is moving directly towards or away from us. (Doppler Effect) Radial velocities are measured using the Doppler Shift of the star's spectrum: Star moving towards Earth: Blueshift Star moving away from Earth: Redshift Star moving across our line of sight: No Shift In all cases, the Radial Velocity is Independent of Distance. Earth
Tangential Velocity Over a period of time, a star will have moved across the sky a distance. Divide that distance by the time and get the Velocity and also measure the Proper Motion Angle.. Tangential Velocity (v t ). where: = Proper Motion in arcsec/yr d = Distance in parsecs The formula above gives v t in km/sec.
Each of these velocities forms the legs of a right triangle with the true space velocity (v) as the hypotenuse. We can then use the Pythagorean Theorem to derive the True Space Velocity (v):
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