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Page 1: Stars print

Stars

Page 2: Stars print

The Solar Neighborhood

Nearest star to the Sun: Proxima Centauri

which is a member of a 3-star system: Alpha Centauri complex

Model of distances:

Sun is a marble, Earth is a grain of sand orbiting 1 m away

Nearest star is another marble 270 km away

Solar system extends about 50 m from Sun; rest of distance to nearest star is basically empty

Page 3: Stars print

The Solar NeighborhoodThe 30 closest stars to the Sun:

Page 4: Stars print

Luminosity and Apparent BrightnessApparent brightness is how bright a star appears when viewed from Earth; it depends on the absolute brightness but also on the distance of the star.

Absolute brightness, or luminosity, is a measure of the total power radiated by a star.

Therefore, two stars that appear equally bright might be a closer, dimmer star and a farther, brighter one:

Page 5: Stars print

Luminosity and Apparent Brightness

Apparent brightness is measured using a magnitude scale, which is related to our perception.

It is a logarithmic scale; a change of 5 in magnitude corresponds to a change of a factor of 100 in apparent brightness.

It is also inverted – larger magnitudes are dimmer.

Page 6: Stars print

Luminosity and Apparent BrightnessApparent magnitude is a measure of the brightness of a celestial object as seen from Earth.

The lower the number, the brighter the object. Negative numbers indicate extreme brightness.

We can see objects up to 6th magnitude without a telescope.

This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C

Page 7: Stars print

Stellar Temperatures

The color of a star is indicative of its temperature:

Red stars are relatively cool, while blue ones are hotter.

Page 8: Stars print

Stellar Sizes

Stellar radii vary widely:

Giant stars have radii between 10 and 100 times the Sun’s.

Dwarf stars have radii equal to, or less than, the Sun’s.

Supergiant stars have radii more than 100 times the Sun’s.

Page 9: Stars print

Mass and Other Stellar Properties

So the most massive stars have the shortest lifetimes – they have a lot of fuel but burn it at a very rapid pace.

On the other hand, small red dwarfs burn their fuel extremely slowly, and can have lifetimes of a trillion years or more.

Mass is related to stellar lifetime:

Page 10: Stars print

Characterizing Stars

Page 11: Stars print

Hertzsprung-Russel Diagram• The diagram was created

circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represents a major step towards an understanding of stellar evolution or "the lives of stars".

• Plots stars on a graph measuring the star's absolute magnitude or brightness against its temperature and color.

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Giants and Dwarfs

• Giant stars have larger radii and luminosity than a main sequence star.

• Giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun.

• Dwarf star alone generally refers to any main sequence star.

• Lower mass than giants.

• Create energy through the nuclear fusion of hydrogen atoms into helium.

Page 13: Stars print

Yellow Dwarf• A yellow dwarf is a main-sequence

star in the process of converting hydrogen to helium in its core by means of nuclear fusion.

• A yellow dwarf has about 0.8 to 1.2 solar masses and a surface temperature of between 5,300 and 6,000 K.

• Yellow dwarves actually range in color from white to only slightly yellow

• When the hydrogen is exhausted the star expands to many times its previous size and becomes a red giant.

• Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core cools and contracts into a compact, dense white dwarf.

Page 14: Stars print

Red Giants

• A red giant is a luminous giant star of low mass in a late phase of stellar evolution.

• They have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.

• Formerly main sequence stars.• Will eventually become white

dwarves.

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White Dwarf• A white dwarf is a small star

composed mostly of electron-degenerate matter.

• They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth.

• Luminosity comes from the emission of stored thermal energy.

• Over a very long time, a white dwarf will cool to temperatures at which it will no longer be visible, and become a cold black dwarf.

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Brown Dwarf

• Brown dwarfs are objects with mass too low to sustain stable hydrogen fusion.

Page 17: Stars print

Blue Giant• A blue giant is a massive star

that has exhausted the hydrogen fuel in its core.

• Blue giants have a surface temperature of around 30 000 K and a luminosity some 10 000 times that of the Sun.

• Blue giants are extremely luminous, reaching absolute magnitudes of -5, -6 and even higher.

• A sizable fraction of their energy output is in the ultraviolet range, thus invisible to our eyes.

• As they grow older they expand and cool, eventually becoming red giants.

Page 18: Stars print

The biggest star• VY Canis Majoris (VY CMa) is a red hypergiant star. • 1800 to 2100 solar radii • The largest known star and also one of the most luminous

known. • It is located about 4,900 light years away from Earth.

Page 19: Stars print

Binary Stars

• Two stars orbiting their common center of mass.

• Brighter star: primary• Dimmer star: companion• Most stars are binary

Page 20: Stars print

Binary Stars and Supernovae

• If the companion star is a white dwarf

• The white dwarf can steal mass away from the primary

• When the mass of the white dwarf reaches the Chandrasekar limit…

• Type 1a supernova!

Page 21: Stars print

Neutron Stars and Pulsars

• Giant stars end in Type II supernovae

• The compressed core makes a star of all neutrons : a neutron star

• Neutron stars are highly magnetized and rotate rapidly

• The visible manifestation are called pulsars.

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Nebula

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Nebulae“Nebula” is a general term used for fuzzy objects in the sky.

Two Types:

Diffuse nebula

Planetary nebula

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Diffuse nebula

• Larger than planetary nebulae

• Make up most nebulae

• Two types:– Emission nebulae– Reflection nebulae

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Emission Nebulae• Some diffuse

nebulae occur near an extremely hot, bright star

• Intense ultraviolet light from the star energizes the gas atoms of the nebula and enables the mass to emit light

Page 26: Stars print

Emission Nebulae

Emission nebulae

generally glow red –

this is the Hα line of

hydrogen.

The dust lanes

visible in the image

are part of the

nebula, and are not

due to intervening

clouds.

Page 27: Stars print

Emission Nebulae

Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines:

Page 28: Stars print

Emission Nebulae

Emission nebulae mostly consist of hydrogen, helium, and trace components.

Oxygen is responsible for the greenish color in the Orion nebula:

Page 29: Stars print

Reflection Nebulae

These selections illustrate a reflection nebula and how it forms. Reflection nebulae are usually blue, due to scattering of light.

Page 30: Stars print

Planetary Nebulae

• Planetary nebulae are ball-like clouds of dust and gases that surround certain stars.

• They form when a star begins to collapse and throw off the outer layers of its atmosphere.

• Planetary nebulae are very faint– None visible with the

naked eye