STARS

What is a Star?• Huge, hot, bright balls of gas.

– Example: our sun (medium-sized star that is not very hot); It is the closest star to us.

X-ray Image of the Sun

3-D Image of the sun

UV Image of the sun

Normal Telescopic Image of the sun

Distance from the Sun to Earth = An Astronomical Unit

• The distance from the Sun to the Earth is 93 million miles.

• 93 million miles = 1 astronomical unit.

• 1 astronomical unit = (150 million kilometers)

Proxima Centauri• The next closest star to Earth is Proxima

Centauri– located 4 light years away (9.5 trillion

kilometers X 4 = 38 trillion kilometers away)

• Even with our current technology, which allows space probes to go 25,000 miles per hour (mph), it would take 150,000 years to reach Proxima Centauri.

Astronomical Unit (AU)Planet Average Distance from the Sun

(measured in AU)

Mercury 0.39

Venus 0.723

Earth 1.0

Mars 1.524

Jupiter 5.203

Saturn 9.539

Uranus 19.18

Neptune 30.06

How Are Stars Classified?A. Stars are classified by:

1. Temperature

2. Magnitude (brightness of a star):a. absolute magnitude: actual brightness of a star

(like absolute values in math)

b. apparent magnitude: how bright a

star appears based on its energy

output, distance from you, &

comparison to other stars

(flashlight demo)

Our Sun

• Apparent magnitude of our sun is 26.4, because it is so close. – If it were further from us, it would look much

dimmer.

• Absolute magnitude of our sun is 4.6.– Its brightness compared to the rest of the

stars, if you lined them all up next to each other.

Temperature of Stars

• Scientists can tell the temperature of a star by its color:– Hotter stars tend to be blue– Cooler stars tend to be red.

• HOTS: Our sun is yellow. Is it hot, cold, or in-between?

• The color of stars tells us what elements are in them, such as hydrogen or helium.

• Each element gives off a different color, allowing scientists to know what a star is made of based on its color spectrum.

Spectrum = the rainbow of colors making up visible light

http://www.neosci.com/demos/10-1071_Photosynthesis/Presentation_7.html

Hydrogen Spectrum

http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-spectroscopy.html

Helium Gas Spectrum

http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spectra/background-spectroscopy.html

Betelgeuse-Red StarConstellation Orion

BetelgeuseBetelgeuse: 3500 * C

Rigel- Blue StarConstellation Orion

Rigel

20,000 *C

What color is a star really? Can you tell?

• It is difficult to distinguish between colors at low levels of light because of the way the eye is made. – Two types of cells, rods and cones, enable you to see

color and differences in color.

– Rods distinguish shades of color while cones distinguish color in general.

– Cones do not work well with low light, so one is not easily able to distinguish between colors of stars.

HOTS: Using the table above or on p.585, answer these questions:

1. How are the stars arranged?

2. What is the hottest star?

3. What are the coolest?

4. Find our sun and describe its temperature relative to other stars.

Types of Stars

Class Color Temperature inDegrees Celsius

Elements detected Example stars

O Blue Above 30,000 Helium 10 Lacertae

B Blue-white 10,000-30,000 Helium and hydrogen Rigel, Spica

A Blue-white 7,500—10,000 Hydrogen Vega, Sirius

F Yellow-white 6,000-7,500 Hydrogen & heavier elements

Canopus, Procyon

G Yellow 5,000-6,000 Calcium & other metals The Sun, Capella

K Orange 3,500-5,000 Calcium & molecules Arcturus, Aldebaran

M Red Less than 3,500 Molecules Betelgeuse, Antares

Constellations: patterns of stars seen in the sky

• 88 of them; based on Greek & Roman mythology; grid system that contains all stars in our solar system; 200 billion stars in the Milky Way

• Constellations seen from Earth change during different seasons because the Earth is in a different place in space.

– Analogy: a road trip from here to Montgomery: You see different cities on your trip just as you see different constellations on the Earth’s trip around the sun.

• Different hemispheres also see different constellations because they see different parts of the sky based on their locations.

Different types of stars: Classified by size, mass, brightness, color,

temperature, spectrum, & age

Types include main-sequence stars, giants, supergiants, & white-dwarfs

Stars change types through their lives just as you change from a baby to an adult.

Most are main-sequence stars for most of their lives.

The Beginning of Stars Begin as balls of gas and dust

Gravity pulls the gas & dust into a sphere

The sphere gets denser (more tightly packed) and hotter

Heat causes the hydrogen to change to helium = nuclear fusion

The End of Stars Stars burn the gas that makes them as they age

& when stars die much of their gas & dust returns to space.

Stars that lose their gas slowly stay on the main sequence a long time.

Stars that lose their gas quickly stay on the main sequence a short time.

The Life Cycle of Stars

2. Gravity pulls the gas and dust together into a

sphere.

6. A Super giant becomes a:

____________

7. If the mass of the original sphere is very large (6 times the sun), then the supernova forms a _______________.

3. As sphere becomes denser, it gets hotter

and hydrogen changes into helium in a process called nuclear fusion.

4. The high mass sphere becomes

a:_____________

5. The giant becomes a:

____________

1. A high mass star starts as a ball of gas and

dust.

Possible answers:Giant, supernova, super giant, black hole

The Lifecycle of a High Mass Stars: lose their gas quickly & stay on the main sequence a short

time.

2.Gravity pulls the

gas and dust together into a

sphere.

6. A white dwarf becomes a:

___________

1. Low mass star

starts as a ball of gas and dust.

3. As sphere becomes denser, it gets hotter

and hydrogen changes into helium in a

process called nuclear fusion.

4. The low mass sphere becomes

a:_____________

5. A giant becomes a:

____________

Possible answers:Giant, dark dwarf, white dwarf

The Lifecycle of a Low Mass Star (such as our sun): lose their gas slowly & stay on the

main sequence a long time.

Star Types

Star types are based upon their size.

Largest stars; can be up to 1,000 times the size of

the sun.

Large stars. Size is 10 times to 100

times the size of the sun.

This group includes the Sun.Smallest stars

Size of these stars ranges from 1/10 of the sun’s size to 10 times the sun’s size.

If a super giant star replaced our sun in our solar system, its size would cover Earth, Mars,

Jupiter, and Saturn.

Star types:GiantsDwarfsSuper giantsMedium-sized stars

red dwarfs, low mass stars found at the end of the main sequence,

remain there a long time, & are some of the oldest stars in the galaxy

white dwarfs, which are the small, hot leftovers of an

old star. (Ex.: sun)

This group includes the red giants, which are the large, cool leftovers of stars the size of our

sun & larger.

dark dwarfs, which have an iron core & produce no

energy (light or heat)

H-R Diagram: Hertzsprung/Russell The H-R Diagram is a graph that shows the

relationship between a star’s surface temperature and its absolute magnitude.

Graph shows:A. temperature- by color

B. absolute magnitude

C. size

D. class- see chart, spectral classes

H-R Diagram

Spectral Type or Spectrum - Color indicates elements or gases that make up the stars.

3500

H-R Diagram Questions

• Where on the diagram would you find most stars?

• What side of the diagram would you find hot stars?

• What side of the diagram would you find cool stars?

• Where on the diagram would you find white dwarfs?

• Where on the diagram would you find giants & supergiants?

• Where on the diagram would you find red dwarfs?

H-R Diagram Questions• Where on the diagram would you find most stars?

-in the center on the main sequence

• What side of the diagram would you find hot stars?– On the left

• What side of the diagram would you find cool stars?– On the right

• Where on the diagram would you find white dwarfs?– Lower left

• Where on the diagram would you find giants & supergiants?– Upper right

• Where on the diagram would you find red dwarfs?– lower right

H-R Diagram Questions• What is the spectral class of a star with a temperature of 10,000ºC & a

magnitude of +10?

• What is the spectral class of a star with a temperature of 5,000ºC & a magnitude of -2?

• What is the spectral class of a star with a temperature of 7,000ºC & a magnitude of +3?

• What is the spectral class of a star with a temperature of 10,000ºC & a magnitude of +10?

• What is the spectral class of a star with a temperature of 3,500ºC & a magnitude of -9?

• Which star is a giant?

• Which star is a white dwarf?

• Which star is a supergiant?

• Which star is most like the sun?

H-R Diagram Questions• What is the spectral class of a star with a temperature of 10,000ºC & a

magnitude of +10? A

• What is the spectral class of a star with a temperature of 5,000ºC & a magnitude of -2? B

• What is the spectral class of a star with a temperature of 7,000ºC & a magnitude of +3? C

• What is the spectral class of a star with a temperature of 3,500ºC & a magnitude of -9? D

• Which star is a giant? B

• Which star is a white dwarf? A

• Which star is a supergiant? D

• Which star is most like the sun? C

Main-sequence stars After a star forms, it enters its 2nd & longest part of its life—

main sequence

Hydrogen fuses together to make helium, releasing large amounts of energy—much in the form of heat & light.

Stars that lose their gas slowly stay on the main sequence a long time.

Stars that lose their gas quickly stay on the main sequence a short time.

Giants & Supergiants 3rd stage of a star’s life = red giant

Goes to this after leaving the main sequence because it has used most of its hydrogen

The star continues to cool after leaving the main sequence, forming a red giant (10X sun) or red supergiant (10 to 100X the sun)

White Dwarfs Final stage of a star’s life cycle

Small, hot star made from the leftover core of a star

Can shine for billions of years before cooling completely

When Stars Get Old & Leave the Main Sequence

Average stars becomes red giants & then white dwarfs (stars like our sun)

Massive stars may explode intensely, creating supernovas, neutron stars, pulsars, & black holes.

Supernovas Blue stars may explode at the end of their

lives creating a supernova.

Supernova = gigantic explosion in which a massive star collapses Explosion is so powerful it can be brighter than a

galaxy for days

http://heasarc.gsfc.nasa.gov/docs/snr.html

Neutron Stars & Pulsars Leftovers from supernovas form these

Neutron stars form from the neutrons from the supernovas If the neutron star is spinning, then it is a pulsar.

Pulsars send out beams of radiation that spin rapidly & are detected by radio telescopes as pulses; hence the name pulsar.

Black Holes

Massive leftovers of supernovas collapse to form black holes

They are so massive that light cannot escape them—hence the name black hole.

They don’t gobble up things around them, but will absorb them if they cross the event horizon—the edge of the black hole.

Black holes are difficult to detect unless dust or gas from something nearby spirals into it.