Download What is a Star?

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
Normal Telescopic
Image of the sun
UV 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)
Average Distance from the Sun
Planet
(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_spect
ra/background-spectroscopy.html
Helium Gas Spectrum
http://imagine.gsfc.nasa.gov/docs/teachers/lessons/xray_spect
ra/background-spectroscopy.html
Betelgeuse-Red Star
Constellation Orion
Betelgeuse:
Betelgeuse
3500 * C
Rigel- Blue Star
Constellation Orion
20,000 *C
Rigel
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.
Types of Stars
Class
Color
Temperature in
Degrees 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
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.
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
Possible answers:
1. A high mass
star starts as a
ball of gas and
dust.
Giant, supernova, super
giant, black hole
7. If the mass of the
original sphere is very
large (6 times the
sun), then the
supernova forms a
_______________.
2. Gravity pulls
the gas and dust
together into a
sphere.
The Lifecycle of a High Mass Stars: lose their
gas quickly & stay on the main sequence a short
time.
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:
_____________
6. A Super giant
becomes a:
____________
5. The giant
becomes a:
____________
Possible answers:
Giant, dark dwarf,
white dwarf
6.
A white dwarf
becomes a:
1.
Low mass star
starts as a ball of
gas and dust.
2.
Gravity pulls the
gas and dust
together into a
sphere.
The Lifecycle of a Low Mass Star (such as
our sun): lose their gas slowly & stay on the
main sequence a long time.
___________
5.
A giant
becomes a:
____________
4. The low mass
sphere becomes
a:_____________
3. As sphere becomes
denser, it gets hotter
and hydrogen changes
into helium in a
process called nuclear
fusion.
Star types:
Star Types
Giants
Dwarfs
Super giants
Medium-sized stars
Star types are based
upon their size.
Smallest stars
This group includes the
Sun.
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
Size of these stars ranges
from 1/10 of the sun’s size to
10 times the sun’s size.
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)
Large stars.
Size is 10 times to 100
times the size of the sun.
Largest stars; can be up
to 1,000 times the size of
the sun.
If a super giant star replaced
our sun in our solar system, its
size would cover Earth, Mars,
Jupiter, and Saturn.
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
3500
Spectral Type or Spectrum - Color indicates elements or gases that make up the
stars.
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.