Leaving Home

! How do we know the distances to nearby stars? If a star displays twice the parallax, is it twice as far away?

! What is a parsec (pc)? Is it larger, smaller, or about the same size as a light year?

! If a star has a larger magnitude, is it brighter or fainter?! How are apparent and absolute magnitude different? If a

star is 5pc away is its apparent magnitude larger than its absolute magnitude? How about 10pc away? 20pc?

! What are the 7 spectral types in order of temperature?! What is the H-R diagram? How does it record Spectral

Type? Luminosity? Absolute Magnitude? Color? Temperature? What is the Main Sequence?

Learning Objectives

The numbers on this slide are magnitudes

more about these later

Parallax! How do astronomers measure

the distances to nearby stars?

! Parallax is half the angle a star shifts against the backdrop of very distant stars over 6 months of the Earth’s orbit

! 1 parsec (1 pc) – The distance at which the radius of Earth's orbit would make a 1 arcsecond angle

! 1 pc = 206,265AU = 3.09 x 1013 km = 3.26 light years

Distance to a star in parsecs = Star’s parallax in arcseconds

1

Parallax and Parsecs

1/2 degree = 1800 arcsec

A dime placed at arm’s length

Closest star to Earth:Proxima Centauri(part of a system of 3 stars)Parallax: 0.77 arcsecondsThis parallax shift corresponds to a dime placed 3 miles away

The Distances to the Stars

Measured Parallax Distance 1 arcsec 3.26 lyr 0.1 arcsec 32.6 lyr 0.01 arcsec 326 lyr 0.001 arcsec 3260 lyr 0.0001 arcsec 32600 lyr

Apparent Magnitude Scale

-30 -20 -10 0 10 20 30

Apparent magnitudes (how bright objects appear in the sky)

Sun -26.7

Full Moon -12.6

Venus at its brightest

-4.4

Sirius -1.46

Betelgeuse 0.5

Naked eye limit 6.0

Pluto 15.1

Hubble Space

Telescope limit 30.0

Objects that appear brighter have small magnitudes

Objects that appear fainter have large magnitudes

(Most of) Our Nearest Neighbors

13.1 Light Years

13.1 Light Years

Why do more distant objects look so much fainter?

Absolute and Apparent Magnitude!Magnitudes are an archaic, confusing system!What is most important for us is the difference

between apparent brightness and absolute brightness and how it relates to distance

! Absolute Magnitude (how bright a star really is, which we’ll relate to luminosity later) is a measure of how much light a star emits

! Apparent Magnitude (which is called flux) is a measure of how bright a star appears in the Earth’s night sky, it depends on luminosity and distance from the star to the observer

Important bad drawingapparent and absolute magnitude to distance

Molecular absorption lines (e.g., TiO)

Hydrogen absorption

linesSodium absorption lines

Iron, magnesium, calcium absorption lines

Spectral Classes

HOT

COLD

!To understand the physical nature of stars, we need to look at their spectra

!7 classes based on spectral lines!Our Sun is a “G” star

The H-R Diagram! In the early 20th century, two astronomers

plotted absolute magnitude vs. spectral class and found a strong correlation

! This allowed them to forever live in infamy by having their names placed in astronomy slides next to the word “diagram”

! The Hertzsprung-Russell Diagram (also called the H-R Diagram)

Cool Star Hot Star One Fish Two Fish Red Star Blue Star Bright Star Faint Star

Important bad drawing

Remember Blackbodies! Higher temperature " brighter, bluer! Lower temperature " dimmer, redder

The H-R Diagram! Stars do not

have random temperatures and brightnesses

! 91% of all stars are on the “Main Sequence”

!But, there are also very bright cool stars and very dim hot stars

Next Time

Life on the Main Sequence