Chapter 27: Stars and Galaxies

H. Thiele

Earth Science

Characteristics of Stars

• Size varies from 20km to 1billion km

• Color varies based on temperature

• Mass ranges between 50X less to 50X more than our sun

• Our sun is an average star

Composition

• Star composition observed through a spectrometer– Separates light into its individual colors– Each color represents a different wavelength– Three types of spectra

• Bright-line• Dark-line• Continuous

• Hydrogen is the most common element in the stars. Helium is the second most common

Star Temperature and Composition

Motion and Distance• Motion

– Circumpolar: stars that never go below the horizon

– Tracking circumpolar stars leaves a curved trail

• Distance– Light Year = 9.5x1012km– Use parallax over a six-month period to

determine distance• Parsec: Parallax second-A unit of distance derived

from the measurement of stellar distances by parallax. One parsec is equivalent to roughly 3¼ light years.

– Cepheid Variables: Brighten and fade at regular intervals

Circumpolar Stars

Light Years Kilometres 9,460,528,404,846Miles 5,878,499,812,499Astronomical Units 63,240Parsecs 0.3066

Light Year Conversion Table

Because a light year is directly related to the time light takes to travel through space, it follows that we look out into the universe we also look back in time. For example, in about the year 5,350 BC, a star in the constellation of Taurus exploded. That star was about 6,300 light years from Earth, meaning that the light from the explosion took 6,300 years to cross the intervening space, and finally reached us in the year AD 1054; that was the date Earthbound observers finally saw the explosion that created the Crab Nebula. This 'lag' is a consequence of the immense distances between the stars - when we look up at the Crab Nebula today, we see it not as it is now, but as it was in about 4,300 BC.

Stellar Magnitude• Magnitude: a way to measure the

brightness of a body in the sky– Absolute: the true brightness of the

object• Would line up the stars all 10 parsecs

from the earth• Abbreviated: M

– Apparent: the brightness as seen from earth

• Difference between a 1 and a 5 is 100x • Brightest star, Sirius, -1.46• Brightest planet, Venus, -4.6• Brightest in night sky, moon, -12.5• Brightest in the sky, Sun, -26.8

1 Sirius -12 Canopus -13 Arcturus -04 Alpha Centauri 05 Vega 06 Capella 0.17 Rigel 0.28 Procyon 0.49 Achernar 0.5

10 Betelgeuse 0.5

The Sky's Brightest Stars

MagnitudesApparent Magnitude Scale

Stellar Evolution

Nebula

LightweightHeavyweight

Nebulaa cloud of interstellar gas and dust. Though they can exist in many forms, nebulae tend to be associated with the beginnings and endings of stars. Nebulae form the raw material for the formation of stars, but are also created by material 'cast off' by stars in their final stages of life.

ProtostarsThe earliest stage in a star's development. A protostar represents a region in which the density of the interstellar medium is increasing due to gravitational effects, and in the process of collapsing to form a true star

Fusion• A star is born when

fusion beginsH + H He

• Fusion is a reaction when two small nuclei come together to form larger nuclei

• The result is the release of a large amount of energy

Main Sequence Star• Composed mainly of Hydrogen • The fusion reactions in its core releases enormous amounts

of energy and forms the element helium • A star in this phase of its life, when it belongs to the 'main

sequence', runs on hydrogen 'fuel', which it will eventually convert entirely into helium.

• The lifetime of a main sequence star is heavily dependent on its mass.

• Our own Sun - a fairly typical main sequence star - has been burning hydrogen for about 5,000 million years, and will probably continue to do so for another 5,000 million.

• More massive stars, though, have much shorter lifespans, often measured in mere hundreds of millions of years.

Red Giant• As the star's hydrogen supplies run out, its form changes

significantly. • Its core, now composed almost entirely of helium, begins to

collapse upon itself, releasing further energy. • This is sufficient to power an expansion of the matter

around the decaying core, and the outer layers of the star swell to many times their original size.

• Meanwhile, the collapsing helium core reaches a point where fusion can proceed once again, this time fusing atoms of helium to produce carbon and oxygen.

• In this new phase, the temperature of the outer layers of the swollen star has cooled to give it a red light, and the resulting star type is known as a red giant.

Red Giants

Antares

Planetary Nebula• When a red giant reaches the end of its life, it

casts off its outer shell of matter while its core collapses into a white dwarf.

• The shell expands outwards at incredible speeds forming a distinctive type of nebula the Planetary Nebula.

• Planetary Nebulae can take the form of simple rings or 'bubble' shapes (like the Ring Nebula in Lyra), or more complex spiralling forms, such as that shown by the Cat's Eye Nebula in Draco.

Planetary Nebula

                                  

Cat’s Eye Nebula

Ring Nebula

Dwarfs

• The decaying remnant of a star that has exhausted all available sources of nuclear fusion. They burn very hot.

• White dwarfs are typically just a few times more massive than the Earth

• A white dwarf has no internal energy source, and will eventually lose what energy it has, becoming completely a dead star known as a brown dwarf.

White Dwarfs

H-R Diagrams

The Path Our Sun Will Follow

Type 1 Supernova

• Happens in binary star systems

• Need a brown dwarf and a red giant

• The brown dwarf takes gases from the red giant and undergoes fusion once again

1987A

Stellar Evolution of a Massive Star

Super Red Giants (Super Giants)

• Work the same way as red giants, but more massive.

• They grow larger, cooler, and brighter

• Burn out quicker

Supernova

• Stars which are 5 times or more massive than our Sun end their lives in a most spectacular way; they go supernova.

• A supernova explosion will occur when there is no longer enough fuel for the fusion process in the core of the star to create an outward pressure which combats the inward gravitational pull of the star's great mass.

• In less than a second, the star begins the final phase of gravitational collapse.

• The core temperature rises to over 100 billion degrees as the iron atoms are crushed together.

• The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in an explosive shock wave.

• As the shock encounters material in the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes.

• The shock then propels the matter out into space. The material that is exploded away from the star is now known as a supernova remnant.

Neutron Stars

• Neutron stars are born during supernova• A neutron star has roughly the mass of our Sun

crammed in a ball ten kilometers in radius. • Its density is therefore a hundred trillion times

the density of water; at that density, all the people on Earth could be fit into a teaspoon!

• Because of its small size and high density, a neutron star possesses a surface gravitational field about 300,000 times that of Earth.

Size of a Neutron Star

Pulsars

• Radio pulsars, the first observed in the Crab nebula in the late 1960s

• Believed to be neutron stars that spin at velocities of up to 600 revolutions a second

• Send a beacon of radio waves whirling across space.

Crab Nebula from Supernova of 1054AD

Black Holes• Black holes are formed when an extremely

massive star dies in a supernova

• black hole is a region of space in which the matter is so compact that nothing can escape from it, not even light

• The "surface" of a black hole, inside of which nothing can escape, is called an event horizon.

• The matter that forms a black hole is crushed out of existence

Black Hole Images

The Constellations

What are Constellations

• They are not real– They are images created by people in

the past– They represent everyday objects or

mythical people from ancient times

• They break up the sky into parts that allow us to find our way around

Is the Big Dipper a Constellation?

• NO• The Big Dipper is an asterisim.• Asterisim:

– Familiar groupings in the sky– Can be parts of one or many constellations– Examples: big dipper, little dipper, summer

triangle, northern cross

What Constellations Should I Know?

• Cassiopeia

• Cepheus

• Cygnus

• Orion

• Ursa Major

• Ursa Minor

The End