Chapter 21Chapter 21

STARSSTARS

Characteristics of StarsCharacteristics of Stars• Groups of stars that form patterns in the

sky are called constellations– Example: Ursa Major (Big Bear), Ursa Minor

(Little Bear), and Orion

• The last two stars in Ursa Major’s “dipper” are called the “Pointer Stars” and are be used to find Polaris (North Star)

• Polaris is located directly above the North Pole (90º N), and is only visible in the northern hemisphere (above the Equator)

Circumpolar Circumpolar ConstellationsConstellations

• Because of the Earth’s rotation, the constellations appear to move

• If the constellations 1) appear to move around Polaris and 2) can be seen at all times of year and 3) can be seen at all times of night, they are called circumpolar constellations – The constellations Ursa Major and Ursa

Minor are both circumpolar constellations

• Using time exposure photography, the apparent motion of the stars around Polaris can be recorded as circular trails

VIFVIFThe apparent motion of stars is due to the

Earth’s daily rotation on its axis.

The stars don’t move – WE DO!!!!

                                                                                                                                                                                                                    

Here is a time-lapse photo of circumpolar star movement…

Star trail photos!!!!!

•The positions of the constellations as viewed from Earth changes from season to season

•This is caused by the revolution of the Earth and the change in Earth’s position in its orbit around the sun –Example: Orion the Hunter is a winter constellation

Ex – when the Earth is in this position (Nov 21), the bright sun during the day blocks our view of all of the constellations toward the lower right side of the diagram

Physical Properties of Physical Properties of StarsStars

• Stars differ in size, density, mass, composition, and color

• The color of a star is determined by it surface temperature (ESRT’s P. 15 top) – The hotter the star, the bluer the

color. The cooler the star, the redder the color. (yeah, yeah, I know, it’s backwards….)

– The sun is an AVERAGE SIZE, medium, yellow star

• Most stars are made up of mostly hydrogen and helium (approx. 98%)

• The remaining 2% may be other elements

• A spectral analysis (remember Ch. 20) of the star can tell us what elements a star is made of, since the radiated spectrum depends on a star’s composition and temperature

Physical Properties of Physical Properties of StarsStars

•Some stars may appear to be brighter than others

•The star’s brightness may be described in three ways

1. APPARENT MAGNITUDE2. LUMINOSITY3. ABSOLUTE MAGNITUDE

(See the H-R Diagram in the ESRT’s P.15)

ApparentApparent MagnitudeMagnitude

• How bright a star appears (apparent) to us on Earth

• The farther a star is from Earth (increasing distance), the dimmer it will look even though it may actually be a very bright star– Because of this, apparent magnitude

does not tell the true brightness of a star

LuminosityLuminosity •The actual (true) brightness of

the star•Depends on the size and

temperature of the star•Hotter stars are more luminous

(brighter) than cooler stars • If the temperatures are the

same, a larger star will be more luminous

Absolute Absolute MagnitudeMagnitude

•The luminosity of the stars if they all brought to the same distance from Earth–aka – picture all the stars lined up the same distance from Earth, then compare their brightness

•This is the most useful when comparing the brightness of the stars

The H-R Diagram

Distances to the Distances to the StarsStars

• The sun is the closest star to Earth• It is approx. 150,000,000 km (93,000,000

miles) from the Earth– This distance is called an astronomical unit

(AU)

• The next closest star to Earth, after the sun, is Proxima Centauri– It is 300,000 times farther away from Earth

than the sun. Because of the great distances in space, larger units of measure must be used

• The light-year is the distance that light travels in one year

• Since light can travel 300,000 km/sec (186,000 miles/sec), light travels 9.5 trillion km/year!!! – Proxima Centuri is 4.3 light-years from Earth!

So…

One Astronomical Unit (AU) = 150,000,000 km

And, one light year (LY)= 9.5 trillion km

(9,500,000,000,000 km)

Okay… let’s calculate the Okay… let’s calculate the distances from Earth to each distances from Earth to each planet in Astronomical Units planet in Astronomical Units

(AU)(AU)

Remember – 1 AU = 150,000,000 kmJust divide the distance from the

Sun in km by 150,000,000 km.

Example: Jupiter = 778,300,000 km 150,000,000 kmJupiter is 5.19 AU from the Sun

Star Origin,Star Origin,FormationFormation

& & EvolutionEvolution

• large clouds of dust and gas in space are the basic materials needed for star formation

• the majority of this gas is hydrogen • some outside force causes the cloud

of gas and dust to be pushed together

• as the gas and dust get closer, friction between the particles causes the temperature to increase

• the attraction of gravity between the particles causes them to continue to move together, and density also increases

• friction increases and temperature increases until the center becomes so hot that nuclear fusion takes place

• hydrogen atoms are forced together to form helium atoms, and a tremendous amount of energy is released

• In a nuclear reactor like Indian Point, nuclear fission takes place – This is when radioactive atoms are

split apart to release energy

OK, so stars form from OK, so stars form from hydrogen gas and dust, hydrogen gas and dust, but where does that gas but where does that gas

& dust come from????& dust come from????

SUPERNOVASSUPERNOVAS• One of the most energetic explosive

eventsoccur at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy

• If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy

• This will cause a blast wave that ejects the star's gas envelope into interstellar space

                         Etna Carinae

Supernova ringsSupernova remnant

SUPERNOVA 1987 – right image is the star that became the left image after going supernova – shone brighter

than most galaxies for a few months!

NEBULAENEBULAE• Clouds of dust & gas (supernova

remnants?)• 2 Main Types:

– Diffuse Nebula – nearby star illuminates the gas/dust cloud

– Dark Nebula – Dark patch against more-distant stars (dust/gas is blocking the light from stars behind it)

Here are some images of nebulae, courtesy of our friend Hubble…

ORION NEBULA

KEYHOLE NEBULA

HELIX NEBULA

VEIL NEBULA

CAT’S EYE NEBULA

CARINA NEBULA

8000 LY FROM EARTH – 200 LY ACROSS

ESKIMO NEBULA

5000 LY FROM EARTH - 10,000 YRS OLD

EAGLE NEBULA

7,000 LY AWAY FROM EARTH

STINGRAY NEBULA

YOUNGEST KNOWN NEBULA – 130 SOLAR SYSTEMS ACROSS – 18,000 LY AWAY FROM EARTH

CRAB NEBULA

HORSEHEAD NEBULA – PART OF THE ORION NEBULA

SNAKE NEBULA – a Black Nebula!!!

A STAR IS A STAR IS BORN…BORN…

I WANT MY MOMMY!!

LIFE CYCLE OF STARSLIFE CYCLE OF STARS

• VIF!!!! - A star’s life cycle is determined by its MASS– The larger the star, the faster it burns

out!

• A star’s MASS is determined by the MATTER available in the nebula of formation

LIFE CYCLE OF STARS

STELLAR NURSERY

STARS FORM IN A NEBULA OF

GAS & DUST

SUN-LIKE STARS

(UP TO 1.5 X MASS OF OUR

SUN)

RED GIANT WHITE DWARF

BLACK DWARF

MASSIVE STARS

(1.5 – 3 X OUR SUN)

SUPERMASSIVE STARS

> 3 X OUR SUN

RED SUPERGIANT

RED SUPERGIANT

SUPERNOVA

SUPERNOVA

NEUTRON STAR

BLACKHOLE

PLANETARY NEBULA(NOVA)

DEATH OF A SUN-LIKE STAR

SUN-LIKE STAR RED GIANT NEBULA WHITE DWARF

BLACK DWARF

LONGEST, MOST STABLE PERIOD OF A STAR’S LIFE –

CONVERTS HYDROGEN TO HELIUM, RADIATING

HEAT & LIGHT

NUCLEAR FUEL

DEPLETES, CORE

CONTRACTS, SHELL

EXPANDS

OUTER LAYERS DRIFT

OFF INTO SPACE IN

SPHERE-LIKE PATTERN

STAR COOLS ARE

SHRINKS BECOMING

ONLY A FEW

THOUSAND MILES

ACROSS!

NO NUCLEAR REACTION

STAR LOSES

ALL HEAT TO SPACE

AND BECOMES

COLD AND DARK

CARBON BALL

GIANTS/GIANTS/SUPERGIANTSSUPERGIANTS

• the brightest & largest kind of star

• luminosities of 10,000 to 100,000 • radii of 20 to several hundred

solar radii (they are about the size of Jupiter's orbit!!!!)

• two types are red supergiants (Betelgeuse and Antares) and blue supergiants (Rigel)

Betelgeuse a red supergiant, with about 20 times the mass and 800 times

the radius of the Sun, so huge that it could easily contain the orbits of

Mercury, Venus, Earth, Mars & Jupiter. It will probably explode as a supernova at some point within the next 100,000

years. Even at its relatively remote distance, it normally ranks as the tenth

brightest star in the sky.

Rigel, a blue supergiant, has a diameter of about 100 million kilometers, some seventy times that of the Sun. Within a few million years, it will probably evolve to become a red supergiant like its neighbor in Orion (though not in physical space), Betelgeuse.

Dwarf StarsDwarf Stars• A term used, oddly enough, to

describe any star that is of normal size for its mass

• The Sun, for example, is classified as a yellow dwarf

• In general, dwarf stars lie on the main sequence and are in the process of converting hydrogen to helium by nuclear fusion in their cores

http://www.phy.mtu.edu/apod/image/9711/m4wds_hst_big.jpg

White DwarfsWhite Dwarfs• A medium sized star that has

exhausted most or all of its nuclear fuel and has collapsed to a very small size

• Typically part of a planetary nebula

• Eventually cools into a BLACK dwarf (lump of carbon) – This takes BILLIONS of years!– This is the fate of OUR SUN!

Neutron StarNeutron Star•The imploded core of a

massive star produced by a supernova explosion

•The most dense known objects in the universe!–A sugar cube size of neutron star material weighs 100 million tons!!!!!!!

BLACK HOLESBLACK HOLES•A supermassive star that

undergoes supernova and the core is when the star is “swallowed” by its own gravity

•readily attracts any matter and energy that comes near it - including light…

3,700 LY wide dust-disk encircling a 300 million solar mass blackhole in the center of an elliptical galaxy.

The disk is a remnant of an ancient galaxy collision and could be “swallowed” up by the blackhole in a few billion

years.

Galaxies & the Galaxies & the UniverseUniverse

Big Bang TheoryBig Bang Theory

• The Big Bang Theory is the dominant scientific theory about the origin of the universe

• According to the big bang, the universe was created sometime between 10 billion and 20 billion years ago from a cosmic explosion that hurled matter and in all directions

Galaxy FormationGalaxy Formation• The formation of all the galaxies is

explained by the Big Bang Theory• Simply put, it states that the

universe was a big ball of hydrogen gas that exploded outward

• The expanding cloud had areas that condensed into galaxies that are still expanding out from the center (the universe is getting larger)– We can see this via RED SHIFT!

The Solar System

GalaxiesGalaxies• system containing millions to billions of

stars – Ex. the Milky Way galaxy contains over

500,000 million stars

• Milky Way galaxy is a spiral shaped galaxy with a large central cluster of stars, and thinner “arms” radiating out from the center – The solar system is located on one of the

arms of the Milky Way galaxy about 2/3 away from the center

Origin of the Milky WayOrigin of the Milky Way

• Formed 10-12 billion years ago• Possibly collided with smaller

galaxies• Globular star clusters formed• Stars and solar systems formed

roughly 5 billion years ago

Milky Way

Andromeda Galaxy – 2.9 million LY from earth

Our CLOSEST neighbor!!!

Ursa Major’s galaxy (a galaxy that we can see within the constellation) –

50 million LY from earth!

Sombrero Galaxy – 50 million LY from earth – 100,000 LY across

Overlapping Spiral galaxies – 140 million LY from earth – within the constellation HYDRA

Whirlpool Galaxy – 37 million LY from earth – 60,000 LY across

Hoag’s Galaxy – 120,000 LY wide – perfect ring of blue stars surrounding older nucleus

of yellow stars

When galaxies collide…