� The light from the stars we see

at night travel great distances

to reach Earth.

� Other than the sun, Proxima

Centauri is the next closest

star. star.

� The light that we see from it

takes over 4 years to reach our

eyes.

� Other light from

stars has taken

thousands,

millions, or even millions, or even

billions of years to

reach Earth.

The stars of Orion's belt: Mintaka, Alnilam and

Alnitak. (800-1300 ly)

� When ancient people

looked at the sky, they saw

pictures of mythic heroes

(Hercules), animals (Draco)

and objects (Lyra).

� They used to tell stories � They used to tell stories

about the groups of stars

in the sky, which are now

called constellations.

� 88 different

constellations can

be seen from the

Earth’s northern &

southern

hemispheres.

� Constellations are � Constellations are

not natural; they are

man’s grouping of

stars as we view

them from Earth.

� The Big Dipper is

probably the best

known asterism – a

small grouping of stars

from a larger from a larger

constellation.

� The Big Dipper

is part of the

constellation constellation

Ursa Major or

the Great Bear.

The dipper can be used to find other

constellations.

� Draw a line through the two stars farthest from the

dipper’s handle.

� The line through these “pointer stars” leads to the

last star in the handle of the Little Dipper.

� This is Polaris, or the North Star.

The movement of the stars in the sky is due the Earth’s rotation and revolution.

1. The Earth rotates from

West to East (rule of

thumb)thumb)

� So the whole sky

appears to move from

East to West.

The sun and the stars and the moon rise in the

east & set in the west.

2. One star seems to

be stationary – the

North Star called

Polaris.

This is because it lies This is because it lies

directly above the

North Pole.

All other stars appear to revolve around it.

3. Stars or constellations

close to the North Star

can be seen all year.

They are called

circumpolar stars or

constellations.

4. A constellation’s position in the sky changes

with the seasons.

Some constellations can only be seen during

certain seasons.

During the winter

months, Orion can

be seen from the

Northern

Hemisphere.

• But six months later, the Earth has moved to

the other side of the sun.

• Orion cannot be seen because it is facing the

daytime side of the Earth.

Now people on Earth can see Lyra, a summer

constellation.

Summer - The Teapot, part of Sagittarius, crosses the night sky during the summer. When we look at the Teapot, we are looking towards the center of our Milky Way galaxy

Fall - Pegasus and Andromeda fly near Cassiopeia the Queen. Looking south of and far beyond the Milky Way, we see the distant Andromeda galaxy

Winter - Looking out through the spiral arms of the Milky Way galaxy we see Orion the Hunter chasing Taurus the Bull.

Spring - When Leo the Lion crosses the sky at night, we see galaxies north of our Milky Way galaxy, like the group called the Leo Triplet

Ursa Minor – the little dipper

Distances in Space

� Units we use to measure distances on Earth are way too small for the extremely large distances in space.

� The distance between the Earth and the Sun for example is 150 million kilometers or 93 for example is 150 million kilometers or 93 million miles.

Now that’s a BIG number!

Here’s something easier to remember…

1 AU (Astronomical Unit)

= 93 million miles

= the distance from the Earth to the

Sun

It is easy to measure distances

in our solar system with AU’s,

but what about for far away

stars and galaxies?

� Our closest star, Proxima Centauri is… � Our closest star, Proxima Centauri is…

24.8 trillion miles or 260,000 AU away

from Earth!!

Now that’s another BIG number!

☺ let’s make it easy again…

� Proxima Centauri is 4.2 light years

away

� What is a light year?� What is a light year?

� A unit that measures the distancelight travels in one year

� Light travels 9.5 trillion kilometers in

one year (5.8 trillion miles)

Light travels 300,000 km/sec so…

� Sunlight reaches Earth in 8 minutes

� Sunlight reaches � Sunlight reaches Pluto: 4 hours

� Light from ProximaCentauri reaches Earth: 4.2 years

How far away is Proxima Centauri?

� 24.8 trillion miles or 260,000 AU or 4.2 light years

� It is much simpler to speak & write in light-years!!

How do astronomers measure the distance to stars?

� One method astronomers use to measure distances to stars is called parallax.

You try it…You try it…

� If you hold your finger in front of your face and close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift " with respect to more distant objects behind it.

Parallax

� Stars that are close to Earth show parallax as Earth moves from one side of the sun to side of the sun to the other.

� Astronomers can then determine the distance using geometry!

� Luminosity is another word for the

brightness of a star.

� Ex: different watt light bulbs.

sz� A star’s luminosity depends on size

and .

� The BIGGER the star - the brighter the star.

� The hotter the star - the brighter the star.

When you look up at the stars at night, it is obvious that some are brighter than others.

� However this is deceptive since the observed brightness of a star depends on how far away you are from it.

Astronomers express luminosity of a star through a Astronomers express luminosity of a star through a scale of magnitudes.

is the measure of the brightness of a star.

� Every star needs 2 different magnitudes Magnitude and Magnitude.

� How bright a star appears to us from Earth(comparing which star looks brighter in the night sky)

� We still use the Greek system of apparent .

� The brightest stars were said to be 1st

magnitude.

� The dimmest stars visible in a dark sky far from a city are 6th magnitude. � The dimmest stars visible in a city are often only 3rd

magnitude.

� Later, it became necessary to assign to some stars a magnitude brighter than 1.0.

� Ex: Vega has magnitude of 0 and Sirius has a magnitude -1.4, the sun has a magnitude of -26.74.

� So, the more negative the value of apparent magnitude, the brighter the star appears. magnitude, the brighter the star appears.

� Conversely, the larger the value of magnitude (more positive) the fainter the star appears.

� How bright a star would be if all stars were the

same distance from Earth (32.5 light years).

� The star that is actually brighter comparing

them side-by-side.

� The absolute magnitude of the Sun as about � The absolute magnitude of the Sun as about

+5.

� The absolute magnitude of Rigel is about -7,

meaning it is really incredibly bright.

ColorColorColorColor Surface Temperature (C)Surface Temperature (C)Surface Temperature (C)Surface Temperature (C) ExamplesExamplesExamplesExamples

Blue over 25,000 10 Lacertae

Blue 11,000 - 25,000 RigelSpica

Blue 7,500 - 11,000 Sirius, Vega

Blue to White 6,000 - 7,500 Canopus, Procyon

White to Yellow 5,000 - 6,000 Sun, Capella

Orange to Red 3,500 - 5,000 Arcturus, Aldebaran

Red under 3,500 Betelgeuse, Antares

A light echo around the star V838 Monoceros.A light echo around the star V838 Monoceros.A light echo around the star V838 Monoceros.A light echo around the star V838 Monoceros.•Not quite yet understood. Not quite yet understood. Not quite yet understood. Not quite yet understood.

Red Giants

High LowTemperature

The H-R diagram plots surface temperatures of stars against

their absolute magnitudes.

White dwarfs

Main Sequence Stars

� Where a star begins its life (fusion of H to He)

� Majority of all stars lie in a band that runs diagonally through the

diagram

� Theme: hotter the temp – brighter the star

Giants and Super GiantsGiants and Super Giants

� A group of cool stars that are very bright because of the large size

� Found in the upper right corner of the diagram

White Dwarfs

� Near the end of their lives

� A group of hot stars that are dim because of their small size

� Found in the lower left corner of the H-R diagram