The Sun

• Age 5 billion years • 40 % in the core, 60 % . Used to be 73 % and 25 %. 5 % hydrogen used. • Inner structure

– radiative core until 0.7R⊙ – convective mantle • Sun – closest • Energy created in fusion reactions at a small region in the core • Proton-proton-reaction: 600M tons of hydrogen fuses into 596M tons of helium. The difference into energy (E=mc2). • The power of the Sun is 4 x 1026 W • Mass loss less than 0.1% during the main sequence • Rotation – differential rotation • maintained by the movements of the gas in the convective zone • The rotation of the convective zone studied by using vibrations • Radiative zone rotation – uncertain results – Can rotate like a solid object

• – visible surface – thickness 300 – 500 km – drops from 8000 K to 4500 K – granulation • – sphere of color – thickness 500 km – temperature increases from 4500 K to 6000 K – spicules – Apparent during eclipses • Corona – temperature 106 degrees – solar wind in the outer layers

• Sunspots – structure • Outer penumbra, temperature 5200 K • Inner umbra, temperature 4500 K – period 11 years, changes between 7 – 15 years – Magnetic period • Other activity – facula – plage-regions – protuberance – flare-bursts

Binary

• Classification according to how it was observed – Visual binary stars – Astrometric binary stars – Spectroscopic binary stars – Photometric binary stars • Classification according to the distance between the stars – Wide binary stars – Close binary stars • contact binary stars

Variable stars

• Stars whose brightness changes remarkably during decades • First variables found at the end of the 16th century • Variable names (F. Argelander’s suggestion) – R…Z, RR…ZZ, AA…QZ, then V335 and so on • over 4000 known variables

Pulsating variable stars

• Stars’s size and temperature changes – Brightness changes periodically • Periods from less than a day to multiple years – The change in the brightness depends on the period

Eruptive variable stars

• Brightness increases rapidly, decreases slowly • Two subgroups 1. Eruptive variables - eruption takes place in chromosphere or corona 2. Cataclysmic variables - Nuclear reactions on star surface or in the core

1. Eruptive variables

• Flare stars (UV Ceti-stars) - bursts caused by magnetic disturbances • Variable nebula - T Tauri-stars - young stars, moving to the main sequence - FU Orionis-stars - stars being born (?) • R Coronae Borealis-stars (RCrB) - diminish periodically

2. Cataclysmic variables

• Novae – novae – periodical novae – dwarf novae – -like variables • Supernovae – typeI: light, old stars – type II: young, heavy stars

Rotating variables

• The change in brightness due to changing surface temperature – Star has spots • Spot temperature lower than the rest of the surface • The size of the spot can be remarkably large P – Radiation coming from the star changes – Brightness changes • Sometimes the spot can be seen and sometimes not – The rotation of the star changes the situation

Photometric variables

• Binary stars • Stars transit each other periodically • Surface area changes – brightness changes • The brightness of the stars doesnt change, only the light that we observe

White dwarf

• The end stage of Sun-like stars – Mass = Sun mass and size same as the Earth’s

– upper limit for the mass: Chandrasekhar mass: 1.4M⊙ • The radius of is inversely proportional to the mass • No inner energy source, cools down – stays together because of electron gas’s degeneration • Final stage: Black dwarf

Neutron star

• If the mass of the white dwarf exceeds the Chandrasekhar mass, it collapses into a neutron star – neutron star has an upper limit: Oppenheimer-Volkov-mass 2 – 3

M⊙ – also supernova explosions produce neutron stars • Radius 10 km • Electrons have penetrated the cores of atoms – p + e n + ne • Star is a giant neutron ball

Black hole

• If the mass of the core is 2 – 3 M⊙, a black hole will be born – matter collapses into one point, singularity • Mass determines a distance – event horizon (edge of the black hole) • The distance from the singularity to the event horizon is called the Schwarzschild radius • Black holes dont have hairs • Three quantities last in the collapse – Gravitation – Angular momentum – Electric charge • rotating black hole

Planetary nebulae

• Expanding gas clouds surrounding a small hot blue star • Last phase of Sun-like stars – outer layers expand 20 – 30 km/s – core a white dwarf • Lifespan a few tens of thousands of years • Regular shaped • Estimated 50000, found 1000

Star clusters

• Multiple stars in the sky belong in the same cluster • Types – associations • T Tauri-associations • OB-associations – open star clusters – globular clusters

Star associations

• V. Ambartsumjan in 1947 found groups of young stars • Not many stars – fall apart fast – association generations • T Tauri-associations – Sun-like stars, moving to the main sequence • OB-associations – groups of massive stars • Concentrated heavily on the plane of the Milky Way

Open clusters

• Stars from tens to a few hundred • Distance measurements – Main sequence fit – Proper motions • Fall apart slowly because of the Milky Way’s differential rotation

Globular clusters

• Ages 10 – 13 x 109 years – represent the oldest star generation in the Milky Way • Divided into two groups – intermediate and halo population II – differences in the distributions, orbital planes and metallicity • Quite stable – lose stars every round • Differences in metallicity – former ?