Star overview
Properties of Stars
• Do Inquiry activity pg. 699 in Earth
Science book
– Answer the questions in your notebook
Class discussion: What does these mean
about astronomy and how we see stars
Characteristics of Stars
• Color, Temperature and Mass are three
properties of stars
• Color is a clue to a star’s temperature
• Hot stars appear blue because they emit
short-wave lengths
• Red stars are cooler because they emit
longer wavelengths
• Yellow stars like the sun emit wavelengths
between 5000-6000 K
Temperatures of the Sun and
Stars
• We determine the temperatures of stars
from an analysis of their light (spectra).
Both the continuous and absorption
line spectra of stars can be used to
infer the surface temperatures of stars.
• Consider the spectrum of the Sun made
using a prism:
Sunlight through a prism
Binary star motion demonstration
• What does the first demo represent?
• What will happen if I replace one ball with
a smaller ball?
• What does the second model represent?
Binary stars and Stellar mass
•
•
•
•
Pairs of stars are called binary
Pulled toward each other by gravity
>50% of stars are binary or multiple
Binary stars are used to determine star’s
mass
• The mass of a body can be calculated if it
is attached by gravity to another body
Binary Star Motion
– Binary stars orbit around a common point
called “center of mass”
– The center of mass lies in the middle when
the mass of each body is equal
– If one star is larger the center of mass will lie
closer to the bigger star
– The star masses are then determined by the
size of the orbits
Video Field Trip
Answer the following after viewing the
film
1. What happens when a star runs out
of fuel?
2. Describe what will happen to the
sun when it dies?
Parallax
• Basic way to measure star distances
• Shifting of the position of a nearby star because
of earth’s orbital motion
• A picture of a star is taken, then six months later
another picture is taken and compared. The
nearby star appears to shift against the
backdrop of more distant stars
• Nearer stars have larger parallax angles
Parallax
• What causes the star to appear to shift?
• How is the parallax angle related to the
distance between the star and the earth?
Parallax
• Parallax angles are very small
• Nearest star, Proxima Centauri is less
than 1 second of an arc (1/3600 of a
degree)
• Your finger when your arm is stretched is
about 1 degree!!!
Light Year
• A unit of interstellar distance, defined as the
distance light travels in a period of one year. The
speed of light is constant, at about 300,000 km
per second or 9.5 trillion km/ year: From an
Earthbound perspective, this is a vast distance our entire Solar System, out to the orbit of Pluto,
is only one eight-hundredth of a light year
across.
Meet the neighbors: these are the star systems that lie
within ten light years of the Sun and Solar System. Of
these, only Alpha Centauri and Sirius are visible to the
naked eye: the others are faint red dwarf stars.
Our entire Galaxy is 100,000 light years across or more.
Important points about light years
• A light year is not a time period but a distance
• The sun is a star we don’t measure in light years
because it is only 93 million miles away and its
light only takes 8 minutes to get here!!
• Imagine if the distance between the sun and
earth were an inch than a light year would be
one mile and the star Sirius would be 8 ½ miles
away.
Stellar Brightness
• Magnitude is the measure of a star’s
brightness
• Over 3 billion stars can be seen through
ground based telescopes
• 3,000 can be seen without a telescope
• Hubble telescope can see over a trillion
stars
• Visibility is on the stars brightness and its
distance to earth
Apparent magnitude is the star’s brightness
as it appears from earth.
– How big the star is, how hot it the star is and how far away
from Earth the star is controls the apparent magnitude.
– Astronomers rank stars by apparent magnitude
• the larger the number the dimmer the star
– A star of the first magnitude is about 2.5 x the magnitude of a
star of second magnitude
The faintest star that can be seen without a telescope has an
apparent magnitude of 6 and is called a sixth magnitude star
Some stars, the sun, the moon and some planets have apparent
magnitudes that are in the negative numbers because they are
brighter than first magnitude stars.
The sun has an apparent magnitude of -26.8. The most powerful
telescopes can detect stars with an apparent magnitude of 29
Absolute magnitude
• Absolute magnitude is how bright a star actually is
• Two star with the same absolute magnitude usually do
not have the same apparent magnitude because they
would not be the same distance away
• To compare absolute brightness, astronomers
determine what magnitude the stars would be if they
were 32.6 light-years away.
• The sun’s absolute magnitude is +5, which is middle
range for a star
• Most stars have an absolute magnitude of -5 to +15
• Stars that are more than 32.6 light years away have apparent
magnitudes that are higher than their absolute magnitudes
Vsauce
• http://www.youtube.com/watch?v=FJB7gbj
iJKw&feature=c4overview&list=UU6nSFpj9HTCZ5tN3Rm3-HA
The apparent and absolute
magnitude demo
1. Which flashlight has the greater apparent
magnitude?
Which flashlight has the lower apparent
magnitude?
2. How do the flashlights’ apparent
magnitudes compare?
How do the flashlights absolute magnitudes
compare?
Demo
3. Now which flashlight has the greater
apparent magnitude?
Which flashlight has the greater absolute
magnitude?
What does this mean?
Hertzsrpug-Russell Diagram
• Shows the relationship between the absolute
magnitude and temperature of stars
• Main sequence stars are linear with the hottest
stars being the brightest and the dimmest being
the coolest. The hottest are also the most
massive and the coolest are the least massive
• Red giants are massive, bright but cooler stars
• White dwarfs are small and faint, although not all
are white
Hertzsprung-Russell Diagram
Look at Figure five on pg. 704 in
your book
• To what group of stars does the sun
belong?
• How are absolute magnitude and
temperature related in the main
sequence?
• What types of stars have high absolute
magnitude but low temperatures?
• What type of stars have low absolute
magnitudes and medium temperatures?
Variable stars
• Read pg 705 and answer the following
questions:
• What is a Cephaid variable?
• What is a light period? What is a nova?
• How long are most novas?
• Why do scientists think Novas occur in
certain binary systems?
• Describe the process of a nova flare-up
Building a model
• Work in a group of three to four
• Design a model to represent the different
types of stars-on paper would be fine
– It must be proportional
– The diameters of the stars are:
•
•
•
•
•
Super Giants are about 696 million km
Giants are about 160 million km.
Medium-sized star are about 1.4 million km.
White dwarf are about are about 13,000 km
Neutron stars are about 20 km.
Interstellar matter: Go to Nebula
Power Point
•
1.
2.
3.
4.
5.
Read pg. 706 and answer the following:
What is a nebulae?
What are the two types of nebulae?
Describe an emission nebulae
Describe a reflection nebulae
What are dark nebulae
Star Spectrum and type
Stellar Classification
• stellar classification is a classification of stars
based on their spectral characteristics, what
color of light is given from their photosphere
• We determine this by seeing what atomic
excitations are most prominent in the light,
giving an objective measure of the photosphere's
temperature.
Letters used to represent star type
• O, B, A, F, G, K, and M
• where O stars are the hottest and the letter
sequence indicates successively cooler stars up
to the coolest M class.
• O stars are called "blue", B "blue-white", A stars
"white", F stars "yellow-white", G stars "yellow",
K stars "orange", and M stars "red",
Star Classification
• In the current star classification system, the
Morgan-Keenan system, the spectrum letter also has
a number from 0 to 9 indicating tenths of the range
between two star classes, so that A5 is five tenths
between A0 and F0, and A2 is two tenths of the full
range from A0 to F0. Lower numbered stars in the
same class are hotter.
• Another dimension that is included in the MorganKeenan system is the luminosity class expressed by
the Roman Numbers I, II, III, IV and V, expressing
the width of certain absorption lines in the star's
spectrum