Properties & Types of Stars

Includes blackhole… ….by ANTO

Distances To The Stars

• Stars are separated by vast distances.

• Astronomers use units called light years to measure the distance of stars

• A light-year is the distance that light travels in a vacuum in a year

• Proxima Centauri, is the closest star to the sun.

Measuring_Distance_in_Space__The_Light_Year

Parallax

• Astronomers have developed various methods of determining the distance of stars.

• The change in position of an object with respect to a distant background is called parallax.

• As Earth moves in its orbit, astronomers are able to observe stars from two different positions.

• Astronomers measure the parallax of nearby stars to determine their distance from Earth

Properties of Stars

• Astronomers classify stars by their color, size, and brightness. Other properties of stars are chemical composition and mass.

• Color and Temperature – a stars color indicates the temperature of its’ surface.

• The hottest stars appear blue• The cooler stars appear red• The spectrum of color in a star

is from blues to greens to yellows and reds.

Brightness

• The brightness of a star as viewed from Earth is dependent on many factors such as color intensity and distance.

• Apparent Brightness – is the brightness of a star as it appears from Earth.

• The apparent brightness decreases as its distance from you increases

• Absolute Brightness – is how bright a star really is. The absolute brightness is a characteristic of the star and is not dependent on its distance from Earth.

Composition

• Each star has its own spectrum.

• Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium together making up to 96 to 99.9 % of a star’s mass.

How Stars Form

• A nebula is a large cloud of gas and dust spread out over a large volume of space.

• Some nebulas are glowing clouds lit from within by bright stars.

• A star is formed when a contacting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.

Life Cycles of Stars

• Adult Stars – A star’s mass determines the star’s place on the main sequence and how long it will stay.

• The amount of gas and dust available when a star forms determines the mass of each young star.

• The larger the star the more energy produce.

• Since blue stars burn brightly, they use up their fuel quickly and are short lived.

The Death of a Star

• The dwindling supply of fuel in a star’s core leads to the star’s death as a white dwarf, neutron star, or black hole.

STARS: how they are born, live and die

As the core collapses it gets hotter, though no extra heat has been generated, just because it compresses. It gets so hot that light from the core causes the outer parts of the star to expand and get less dense, whereupon the star looks cooler from the outside.

The star is becoming a red giant.

Electron degeneracy pressure eventually brings the collapse of the core to a halt, before it gets hot enough to fuse carbon and oxygen into magnesium and silicon. The unstable outer parts of the star fall apart altogether; they are ejected and ionized by light from the core, producing a planetary nebula.

Hubble images of PLANETARY NEBULAE

[ Images by the Hubble Telescope - NASA ]

The planetary nebula’s material expands away from the scene in a few thousand years, leaving behind the hot, former core of the star, now about the size of Earth. Its weight supported against further collapse by electron degeneracy pressure, it will do nothing but sit there and cool off, for eternity.

A famous white dwarf: Sirius B

Distance to us = 8.7 light years

Density = 50,000 times that of water

Sirius A

Sirius B

Chandra

X-ray

Observatory

image

(NASA/

CfA

)

How heavy can a white dwarf be?

CHANDRASEKAR LIMIT: 1.4 MSUN

A star heavier than 1.4 Msun can no longer be supported against gravitational collapse by the degeneracy pressure of electrons.

Experimental confirmation of Chandrasekhar’s theory of white dwarfs

Nuclear reactions proceed until core becomes Iron. Core collapses until density becomes so high that neutrons are packed very tightly and their degeneracy pressure supports against gravity

NEUTRON STARNeutron degeneracy pressure prevents further collapse

… as seen by a telescope

[ Supernova 1987A, Anglo-Australian Observatory

]

The explosion is dramatic: supernova SUPERNOVA

• What happens if the Iron core of the collapsing star has a mass larger than 2-3 Msun?

• A BLACK HOLE IS BORN!!

We cannot see black holes directly, but their influence

on the matter around them reveals their presence.

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What is a Black Hole?

Mass is so great in such a small volume that the velocity needed to escape is greater than the speed light travels.

An unimaginably dense region of

space where space is curved around it so

completely and gravity becomes so strong that nothing, not even light, can

escape.

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Where do black holes come from?

Three classifications of black holes: Stellar-mass: 3 to 20 times the mass of our SunSupermassive: Black holes with millions to billions of times the mass of our SunMid-mass: In between stellar-mass and supermassive

Black Holes ain’t black – Stephen hawking…..

• Escape velocity

• Event horizon

• Black hole parameters

• Hawking radiation(general relativity)

Massive bodies and escape speed

Gravity bends the path of light

A nonrotating black hole has only a “center” and a “surface”

• The black hole is surrounded by an event horizon which is the sphere from which light cannot escape

• The distance between the black hole and its event horizon is the Schwarzschild radius (RSch= 2GM/c2)

• The center of the black hole is a point of infinite density and zero volume, called a singularity

• The black hole is surrounded by an event horizon which is the sphere from which light cannot escape

• The distance between the black hole and its event horizon is the Schwarzschild radius (RSch= 2GM/c2)

• The center of the black hole is a point of infinite density and zero volume, called a singularity

Event horizon

• Mass– As measured by the

black hole’s effect on orbiting bodies, such as another star

• Total electric charge– As measured by the

strength of the electric force

• Spin = angular momentum– How fast the black hole

is spinning

Three parameters completely describe the structure of a black hole

Most properties of matter vanish when matter enters a black hole, such as chemical composition, texture, color, shape, size, distinctions between protons and electrons, etc

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How much would you “weigh”?On Earth, let’s say you weigh 150 lbs.

On the Moon, you’d weigh 25 lbs.

On Jupiter, you’d weigh 350 lbs.

On the Sun, you’d weigh 4,000 lbs.

Near a Black Hole, you’d weigh over

20 TRILLON POUNDS !!!

On Earth, let’s say you weigh 150 lbs.

Near a Black Hole, you’d weigh over

20 TRILLON POUNDS

Black Holes Galore!

From the formation of galaxies to the deaths of stars, black holes are an integral part of our universe’s history.

NASA/CXC/SAO/H.Marshall et al.NASA/CXC/MIT/F.K.Baganoff et al.

NASA/CXC/U.Amsterdam/S.Migliari et al.

X-ray: NASA/CXC/MIT/UCSB/P.Ogle et al.Optical: NASA/STScI/A.Capetti et al

NASA/UMD/A.Wilson et al.

Thank you