Stellar Evolution up to the Main Sequence

Stellar Evolution Recall that at the start we made a point that all we can "see" of the stars is: Brightness Color (Spectra) Position Distance (if we are lucky or clever) Let's see if there are any correlations

Stellar Evolution Using distance (when we know it) we can convert the Brightness (apparent magnitude) into the absolute magnitude, or even the Luminosity To make things easy we can write the luminosity relative to that of the Sun, L/L 

Stellar Evolution The Color, or spectra, we can convert to –A Spectral Class –A Temperature –A B-V value V is the visible magnitude B is the magnitude as seen on photographic plates –Photographic plates are more sensitive to blue light – blue stars will appear brighter B-V gives a numerical "Color" index For comparison –the yellowish Sun (G2) has a B-V index of and a surface temperature of about 6000K –the bluish Rigel (B8) has B-V index of and a surface temperature of about 11000K

Stellar Evolution We can plot the Luminosity ratio versus the color: O B A F G K M The Sun would go here

Stellar Evolution This plot was independently discovered by Hertzsprung and Russell It is now called the Hertzsprung-Russell, or H-R, Diagram Ejnar Hertzsprung ( ) Henry Norris Russell ( )

The HR Diagram O B A F G K M The Sun The 50 Nearest Stars

The HR Diagram O B A F G K M The 50 Brightest Stars

The HR Diagram O B A F G K M

The HR Diagram O B A F G K M There appears to be three main areas where the stars are grouped

The HR Diagram O B A F G K M This curve is where 90% of the stars appear

The HR Diagram O B A F G K M These are pretty dim, but also very hot…white hot This implies that they are very small

The HR Diagram O B A F G K M These are cool, but very bright - the size must be huge

HR Diagram O B A F G K M Red Giants White Dwarfs Blue Giants Main Sequence Red Dwarfs

Starbirth

Protostars form in cold, dark nebulae

Distant dark nebulae are hard to observe, because they do not emit visible light However, dark nebulae can be detected using microwave observation, because the molecules in nebulae emit at millimeter wavelengths Giant molecular clouds are immense nebular so cold that their constituent atoms can form molecules. Giant molecular clouds are found in the spiral arms of our Galaxy. Giant Molecular Clouds

Star-forming regions appear when a giant molecular cloud is compressed This can be caused by the cloud’s passage through one of the spiral arms of our Galaxy, by a supernova explosion, or by other mechanisms Giant Molecular Clouds and Star- forming Regions

Molecular Clouds Disorderly Complex Giant Molecular Cloud in Orion Infrared view From IRAS satellite

Molecular or Dark Clouds "Cores" and Outflows Stages of Star Formation Jets and Disks Extrasolar System 1 pc

Stages of Starbirth #  t to Next Stage (yr) Core Temp. (  K) Surface Temp (  K) Diameter (Km) Description 12,000, ,000,000,000,000Interstellar Gas Cloud 230, ,000,000,000,000Cloud Fragment 3100,00010, ,000,000,000Cloud Fragment 41,000,000 3,000100,000,000ProtoStar 510,000,0005,000,0004,00010,000,000ProtoStar 630,000,00010,000,0004,5002,000,000Star 710,000,000,00015,000,0006,0001,500,000Main Sequence

Lifetime on the Main Sequence Luminosity basically describes how fast the star is ‘burning’ its fuel. This is clearly related to how much fuel there is because the greater the mass the higher the pressures and temperatures: L  M 3 Lifetime is “how much fuel / how fast it’s used” T = M/L  1/M 2

Lifetime on the Main Sequence Here are some comparison values: Mass (M sun ) Lifetime (T sun ) Lifetime (years) million million 1110 billion trillion

The Path to the Main Sequence O B A F G K M

The T Tauri phase

Gravity causes the gas/dust cloud to condense. The situation then usually becomes quite complex Some of the infalling gas is heated so much by collisions that it is immediately expelled as an outgoing wind. Jets and disks form as the infalling and outflowing gas collide and interact with changing magnetic fields. Temperatures and masses are similar to the Sun, but they are brighter They have fast rotation rates (few days) Variable X-ray and radio emission Not yet a 'star', but will be in a few million years

During the birth process, stars both gain and lose mass In the final stages of pre–main-sequence contraction, when thermonuclear reactions are about to begin in its core, a protostar may eject large amounts of gas into space Low-mass stars that vigorously eject gas are called T Tauri stars (age ~ 1 million year)

Jets: A circumstellar accretion disk provides material that a young star ejects as jets

Jets: Clumps of glowing gas are sometimes found along these jets and at their ends Known as Herbig-Haro Objects

A Magnetic Model for Jets (Bipolar Outflow)

Starbirth in NCG 281

M16 in Infrared

Bok Globules