Stars and Galaxies

Chapter 27—Stars and Galaxies

What is a Star? Star: A body of gas that gives off a tremendous amount of radiant energy in the form of light and heat

How Do We Know?

What is Light?
A form of electromagnetic radiation which is energy that travels in waves
>Electromagnetic radiation is arranged in a continuum called the electromagnetic spectrum

Continuous Spectrum : all visible colors (wavelengths)
Emission Spectrum : Lines of specific colors (wavelengths)
Absorption Spectrum : all visible colors with specific wavelengths removed forming dark lines

Examples of Spectra

Big Bang Theory
15 billion years ago
Universe compressed into infinitely small space—Primordial Atom
Exploded with sudden burst of energy
Created small, super dense, extremely hot universe
Expanded in all directions
As things cooled, matter clumped forming stars and galaxies
100 million years after “big bang” stars started to form

Evidence supporting Big Bang Theory…
Remnant of initial heat--Cosmic Microwave Background Radiation discovered in 1965 that fills the entire universe
Abundance of Hydrogen and Helium
The size of galaxies relative to their positions.

And Evidence from the Doppler Effect
Edwin Hubble (Hubble’s Law) showed that there is Redshift of star
spectrums which shows that galaxies are moving away from us

Light and the Universe
Light Year —used as a measurement of the great distances in space
A light year is the distance that a ray of light can travel in a year, or:
A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers).
A star or a star system’s distance from earth is measured in light years.

Parallax
A change in an object’s direction due to a change in the observer’s position.
Astronomers also use parallax to calculate the distance to the stars.
Parsec —short for “parallax second”—is a special unit of distance for a star’s distance from earth.
> 1 parsec = 3.258 light years

Why Do Stars Twinkle??
The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation)
Stars (except for the Sun) appear as tiny dots in the sky
As their light travels through the many layers of the Earth's atmosphere, the light of the star is bent (refracted) many times and in random directions
This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling)

Characteristics of Stars Size: anywhere from 20km to 1 trillion km in diameter Mass: the amount of matter Color: stars can be red, blue, white, orange or yellow Composition: contain different elements determined using spectra

Characteristics of Stars Temperature: Blue is the hottest (35000 ° C) Red is the coolest (3000 ° C) Yellow (our sun) (5500°C)

Luminosity —the actual brightness of a star
Depends on size and temperature
Apparent Magnitude —A measure of how bright a star appears to be on earth
the lower the number, higher the brightness
Does not measure how bright a star actually is
Absolute Magnitude —A measure of how bright a star would be if all Stars were at the same distance—ten parsecs—from Earth

0 -10 -20 -30 10 20 30 Magnitudes Dimmer Brighter Our Sun Sirius Brightest in the sky from Earth Polaris/North Star -26.5 -2 3 6 We can not see dimmer than +6 with naked eye

Star 1 High luminosity Far from earth = lower apparent magnitude Star 2 Low luminosity Close to earth = higher apparent magnitude Apparent Magnitude v. Luminosity

Hertzsprung-Russel Diagram
Relationship between the absolute magnitude (luminosity) and the temperature of stars.
Spectral Class —designates the color of stars >depends on temperature >Hottest to the coolest >Blue to White to yellow to orange to Red

Kinds of Stars super giants- largest of all stars, very luminous red giants- cooler, large, very luminous dwarf stars- less luminous, red, orange or yellow white dwarf- very faint, small and dense

Variable Stars- vary in brightness over regular periods or cycles 2 types: A. pulsating change in brightness as they contract (brighter) Or expand (dimmer) Example: cepheid - yellow supergiant

B. nonpulsating Eclipsing Binary 2 stars of unequal brightness that revolve around each other brightness depends on which one is in front of the other

Pulsars star that releases light and radio waves in pulses may be the neutron star formed in a supernova it looks like it pulses because it is rotating , like a searchlight

A star begins it’s life in a cloud of cold gas and tiny-grained dust called a NEBULA .
Parts of the Nebula begin to condense due to some outside force—a shockwave—which acts upon it.
Due to gravity, the gas and dust pockets continue to condense and their temperature increases.
Eventually, parts of the Nebula begins to glow— PROTOSTARS are formed.
Gravitational contraction of the Protostars continues causing them to become hotter and brighter.
Finally, fusion takes place in the center of a protostar, halting gravitational condensation, and a star is born.

Stellar Masses
Expressed as multiples of the mass of the sun—ONE SOLAR MASS
1 solar mass is approximately 2 X 10 30 or approximately 330,000 Earth masses
Solar Mass determines life stages of a star

Life Stages of Stars With Solar Masses less than 8 Solar Masses

Main Sequence Star (Our Sun) — >90% of all stars >The star is stable >Burns Hydrogen gas to Helium in its core through nuclear fusion > The energy released causes the star to shine. >Stars spend about 90% of their active lifetime as main sequence stars. Red Giant — >The star begins to run out of fuel and the core begins to shrink >Helium turns into Carbon >Rapid burning of helium causes outer layer to puff out, cooling the star >The star turns red

Planetary Nebula — >Outer layers are ejected as core continues to shrink >Shell of hot gas >Core is exposed White Dwarf — >Low mass core continues to shrink creating a white dwarf >Surrounded by the Planetary Nebula

Stars Evolution Greater than 8 Solar Masses
Hydrogen fuses more quickly and when a star starts to die, iron nuclei are formed
Star swells to 100 times diameter of the sun— Super Giant
Iron nuclei absorbs energy and core quickly and suddenly collapses
Explodes into a brilliant burst of light— Super Nova
Leaves behind core
> neutron star —(<15 solar masses) dense mass of neutrons
**When 1 st formed, will spin into a
pulsar (pulsations of radiations in regular intervals)
> Black Hole —(>15 solar masses)
a concentration of mass great
enough that the force of gravity
will not allow anything to escape

Star Systems
Closest star system to us is Alpha Centauri which is 4.3 light years away.
The actual star is Proximi Centauri (Alpha Centauri C--4.2 light years away), a red dwarf that cannot be seen with the naked eye
This is a multiple star system. Most are binary.

Novas & Supernovas
Believed to only happen in binary-stars systems.
Gases from one star hit the surface of another and cause a nuclear type explosion.
Supernova is a brilliant burst of light that follows the collapse of the iron core of a massive star.

Super Nova

Star Clusters
There are larger groups of stars, called clusters . These are relatively unorganized collections of stars. (Galaxy)
Open Clusters
Not organized- 100’s of stars
Globular Clusters
Spherical, round- 100,000’s of stars
We live in a star cluster-the Milky way it is a Spiral Galaxy
Spiral galaxies are galaxies with a central, dense area and spiraling arms (which are often sites of star formation).

Other Galaxies
There are several types of galaxies the three main ones are:
Spirals are easily identified by their main components. They are in the shape of a flat disk with a bright central nucleus and spiral arms that extend out from the nucleus.
Ellipticals are different from spirals in that they have very little (or no) gas and dust. They have only stars which are concentrated near their centers
Irregular galaxies have a chaotic appearance, with large clouds of gas and dust mixed with both old and young stars at random. They have no apparent spiral arms or nuclear bulge. Irregulars are generally faint. They make up probably about 25% of all galaxies.

Nebulae
Seen only in infrared
Huge clouds of dust (1%) & gas (99%)
This is where most stars are born.
Stellar nursery is a nebula ( a large cloud of hydrogen gas in space) in which star formation is occurring

Galaxies
Major features if the universe
-100 billion galaxies-each with-100 billions stars.
3 types of galaxies
1. Spiral
2. Elliptical
3. Irregular

3 Types of Galaxies
spiral - nucleus with arms extending out
most galaxies are this type
Milky Way is spiral
2 . elliptical- oval/ disk shaped
3. irregular - smaller, fainter less common

Constellations
Polaris (North Star)
Found at the end of the handle of the Little Dipper (asterism)-which makes up the constellation Ursa Minor (Little Bear)
Also can be found for using the pointer stars in the Big Dipper (asterism)-which makes up the constellation Ursa Major (Big Bear)
Also used for navigatiional purposes

Quasars
Quasars : give off radio & X-waves. They are the most distant objects in space. Give off tremendous amounts of energy.
Quasars give off enormous amounts of energy - they can be a trillion times brighter than the Sun! Quasars are believed to produce their energy from massive black holes in the center of the galaxies in which the quasars are located. Because quasars are so bright, they drown out the light from all the other stars in the same galaxy.

The Constellation, Orion

There are 88 constellations or sky divisions
The constellations change position with each season
Some constellations only come up during certain seasons, some not at all
Some never change position due to their position to the poles— circumpolar
Constellation of Orion is seen during winter

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Stars and Galaxies

by duncanpatti on Apr 18, 2008

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Stars and Galaxies — Presentation Transcript