Transcript Star Properties Star Types HR Diagram

Objectives
• Define luminosity and brightness.
• When you see a bright star, can you tell if it is close? If it is
luminous? Explain.
• Describe how astronomers measure the distance to a nearby
star.
• Describe how to use brightness and luminosity to find the
distance to a star.
• What is an H-R diagram? Sketch one. Given the position of a
star on the H-R diagram, indicate if the star is hot or cold,
luminous or not.
• Roughly describe the luminosity, surface temperature, and
size of the following types of stars: main sequence, giant,
supergiant, white dwarf, red dwarf, brown dwarf.
• Why are supergiant stars rare?
Star Properties
Brightness (B) and Luminosity (L)
Distance to a star (D)
Star sizes
H-R diagram
Luminosity vs Temperature
Some star types
Brightness, B
• *Measure of energy received from a star
– Defined by magnitude
• *Magnitude – visual scale
– from Hipparchus
– 1(bright) to 6(faint)
– Brightest star: -1 is Sirius
Luminosity, L
• *Measure of energy emitted by a star
– Energy per second (Watts)
• *Useful symbol, L
– Luminosity of our sun
Light bulb
• Brightness and Luminosity
• Ex – Two 100 watt bulbs.
– One here and one at coffee shop.
• Which is more luminous?
–Energy emitted
• Which is brighter?
–Energy received
Light bulb
• Brightness and Luminosity
• Ex – Two 100 watt bulbs.
– One here and one at coffee shop.
• Which is more luminous?
–Energy emitted
• Same
• Which is brighter?
–Energy received
• Closer one
Light bulb
• Brightness or luminosity
• Back to 100 watt bulb
• Walk down street with it.
• Brightness depends on distance
• Compare other light bulbs
Bright stars? Magnitude? Very Luminous stars?
Bright stars are: ______________ or ______________
Bright stars are:
Very Luminous and more distant
D
D
L
OR
Close and not so luminous
L
*Brightness, B depends on:
Luminosity, L
Distance, D
L
Bk 2
D
Sirius: B?
Sirius:
B
L?
Sirius:
B
L?
Close (8.6 LY)
Sirius:
B
L
Close (8.6 LY)
Average
luminosity
( 33 L )
Betelgeuse: B?
Betelgeuse:
B
L?
Betelgeuse:
B
Far (429 LY)
L?
Betelgeuse:
B L
Far (429 LY)
Very Luminous!
(330 000 L )
Finding distances to stars
• *Use parallax and
trigonometry on nearby
stars to get distance
(valid for few hundred LY)
Finding distances to stars
For those same nearby stars
• Know D
• Measure B
• Then calculate L
L
Bk 2
D
*Finding distances to stars!!
• Type of star gives you L
• Measure B
• Calculate D
• To read more about L
and types of stars see
Henrietta Levitt
• http://www.windows.ucar.edu/tour
/link=/kids_space/star_dist.html&
edu=high
L
Bk 2
D
*H-R
Diagram
Hertzsprung-Russell
*Star
Luminosity
vs.
Surface
Temperature
*H-R
Diagram
*Star
Luminosity
vs.
Surface
Temperature
H-R Diagram
Which star is hotter?
Which star is more luminous?
Why?
H-R Diagram
Which star is hotter?
SAME (COLOR)
Which star is more luminous?
Why?
LARGER STAR
BECAUSE IT IS LARGER
H-R Diagram
Which star is hotter?
Which star is more luminous?
Why?
H-R Diagram
Which star is hotter?
WHITE STAR
Which star is more luminous?
Why?
SAME SIZE BUT
WHITE IS MORE LUMINOUS
LUMINOSITY DEPENDS ON
TEMPERATURE AND RADIUS
H-R Diagram
Which star is hotter?
Which star is more luminous?
Why?
H-R Diagram
Which star is hotter?
Which star is more luminous?
Why?
BLUE STAR
BLUE STAR
LUMINOSITY DEPENDS ON
TEMPERATURE AND RADIUS
Luminosity depends on…
Temperature and Size (Radius)
H-R Diagram
Which star type can be
hot but not luminous?
Which star type can be
cool but is luminous?
H-R Diagram
Graphing T vs L tells us the size of the star
Which star type can be
hot but not luminous?
WHITE
DWARF
Which star type can be
cool but is luminous?
RED
SUPERGIANT
H-R Diagram
Graphing T vs L tells us the size of the star
Sizes of stars
https://www.youtube.com/watch?v=cMWMtLGGg6o
*H-R
Diagram
Main Sequence
•
•
•
•
•
~90% of all stars
Long lived - Sun ~1 X 1010 yrs
Average, middle class stars
~0.1 to 10 R
~average masses
Main Sequence
Ex: Sun
Main Sequence
Ex: Pleiades
Main Sequence
• Ex: Sun,
Sirius A,
Procyon A,
Alpha Centauri A
Giants (Often red giants)
•
•
•
•
Rare
Shorter lived
~1 - 5 M
~10 to 100 R
Giants
Ex: Arcturus,
Aldebaran,
Spica
Giants
Ex: Arcturus,
Aldebaran,
Spica
Giants
Ex: Arcturus,
Aldebaran,
Spica
Red
Giants
Blue
Giants
Supergiants
•
•
•
•
•
Very rare
Shortest lived
~ 5 to 100 M
~ 10 to 1000 R
Ex: Betelgeuse -- Orbit of Mars or Jupiter
Supergiants
Ex:
 Cephei
Orbit of
Jupiter
White dwarf
•
•
•
•
Mass ~ M
Size ~ Earth (0.01 R)
One tsp weighs ~one ton
Ex: Sirius B
•
•
•
•
•
•
Red Dwarf
Smaller, cooler version of sun
(Not a cooler
white dwarf)
Many
M ~ 0.1 M
R ~ 0.1 R
L ~ 1/2000 L
Ex: Proxima Centrauri
Rho Ophiuchi has 30+ Red dwarfs
H-R Diagram
•
•
•
•
•
Brown Dwarf
Not on HR diagram
Size ~ Jupiter (~1/10 R )
Mass ~ 1/100 M
Failed star (planet?)
10-100 times more
massive than Jupiter
Summary
Brightness and Luminosity
Distance to star
B depends on L and D
H-R diagram (Plot of L vs T)
Types of stars (Main sequence,
Giants, Supergiants, White dwarfs,
Red dwarfs, Brown dwarfs)
Homework & Updates
• Take time to register for MCTC Classes
• No Lab or Lecture on Tuesday, 4/11.
• No Lab this week. Thursday lab students: Lab Exam 1 on
4/13. Tuesday lab students: Lab Exam 1 on 4/18.
• Observations: Star Gazing, Telescopes & Moon Craters (60
pt.)
• (Tonight) April 4 & 6, Tuesday & Thursday, Jackson Middle School,
Champlin, MN
• April 7, Friday, Bell Museum, University of MN, Minneapolis, MN
• Times and directions on class website
• Observation Option: Univ. of MN Public Lectures – Only Do 1!
• TONIGHT: Tues, Apr 4, 7pm, Bell Museum, Exploring the Mysteries of
the Sun: Explosions on our Closest Star (find Raquel for 10 points)
• Thur, Apr 20, 7pm, Ted Mann Concert Hall, A Deeper Understanding
of the Universe from 1.2 miles Underground (find Raquel for 10 points)