Observational Astronomy 4

7/28/2019 Observational Astronomy 4

1/39

19-Apr-13

IESO

Observational Astronomy

Part 4


2/39

2

Star Properties


3/39

3

Apparent Magnitude

System of Hipparchus Group of brightest stars 1m

Stars about as bright as 1m 2m

Stars about as bright as 2m 3m

Naked Eye Limit 6m


4/39

4

Apparent Magnitude

19th century photographers learn how eye responds tolight (Pogson)

Doubling the brightness is not perceived as a doubling by the

eye

Eye response is logarithmic

Ratio of 100 in brightness corresponds to a Difference

of five magnitudes

Dm of 5 100X in light

Dm of 1 2.512X in light


5/39

5

Some Apparent Magnitudes

Sun -26.8

Full Moon -12.6

Venus at brightest -4.4

Sirius -1.5 Naked Eye Limit 6.0

Faintest Objects +30.0

Hubble


6/39

6

Learning the Brightness

Is a star bright... Because it really is a bright star?

Because it is close to the Earth?

Stellar brightness depends on

Luminosity

Distance


7/39

7

Measuring Distance

Stellar Parallax

June

January

Sun


8/39

8

Stellar Parallax

June

January

Sun

1 AU

Parallax


9/39

9

Measuring Parallax

1 AU

1 parsec

1 arcsec


10/39

10

Stellar Parallax

pd

1

When p is measured in arcsec

and d is measured in parsecs

One parsec:

206,265 AU

3.26 light years


11/39

11

Stellar Parallax

Nearest star to Sun (largest parallax) a Cen p = 0.7 arcsec

Limit of accurate parallax 200 pcs (angles of 0.005 arcsec)

Hipparcos satellite (120,000 stars measured to 0.001 arcsec)


12/39

12

Absolute Magnitude

The magnitude a star would have at 10 parsecs from theSun.

The apparent (m) and absolute (M) magnitudes of a star

at 10 pcs are the same.

M, m, and d are related. Knowing two allows you to

compute the third.


13/39

13

Putting the Pieces into Place

Ejnar Hertsprung1911

Henry Norris Russell1913


14/39

14


15/39

15

Luminosity Classes

I Supergiants

II Bright Giants

III Giants

IV Subgiants

V Dwarfs


16/39

16


17/39

17

Luminosity Class implies Size

Consider the Sun and Capella

The Sun

G2V M=5Capella

G2III M=0


18/39

18

Luminosity Class implies Size

Equal sized pieces of each star are equally bright

Capella is 100X brighter (5 magnitudes)

Capella must have 100X as much area

Surface area radius2

Capella must be 10X larger than Sun.


19/39

19

Luminosity Class in the

Spectrum

A3

Supergiant

A3

Giant

A3

Dwarf


20/39

20

Sun G2V

Vega A1V

Betelgeuse M1I


21/39

21

Which of these starsis hottest?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Cant compare


22/39

22

Which of these starsis brightest?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Cant compare


23/39

23

Which of these starsis smallest?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Cant compare


24/39

24

Which of these starsis most distant?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Cant compare


25/39

25

Spectroscopic Parallax

Observe thespectrum and

apparent magnitude

of a star

Classify thespectrum

Plot it on the H-R

Diagram

Read off the M

From m and M

compute

distance

Main

Sequence


26/39

26

Color Index

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0 500 1000

Wavelength (nm)

RelativeEnergy

B V

12000 K

7000 K

*

*

* *


27/39

27

Color Index

Star Temperature mB mV .

1 12000 K 2.0 2.4

2 7000 K 3.0 3.1

Color Index = mB - mV = B-V

1 B-V = 2.0 - 2.4 = -0.4

2 B-V = 3.0 - 3.1 = -0.1


28/39

28

Spectroscopic Parallax

Can now get distances to any object whosespectrum can be measured.

Limit 5000 pcs


29/39

29

Study Tools

Review 1

Review 2
http://astro.unl.edu/classaction/loader.html?filename=animations/stellarprops/spectroparallax.swf&movieid=spectroparallax&width=870&height=600&version=6.0.0http://astro.unl.edu/classaction/loader.html?filename=animations/stellarprops/hrexplorer.swf&movieid=hrexplorer&width=805&height=665&version=6.0.0http://astro.unl.edu/classaction/loader.html?filename=animations/stellarprops/hrexplorer.swf&movieid=hrexplorer&width=805&height=665&version=6.0.0http://astro.unl.edu/classaction/loader.html?filename=animations/stellarprops/spectroparallax.swf&movieid=spectroparallax&width=870&height=600&version=6.0.0


30/39

30

The Advantage of Color Index

Measures temperature just like Spectral Type Much easier to obtain

requires two measurements of brightness

spectral type requires getting the spectrum


31/39

31

Color-Magnitude Diagrams

M

Spectral Type

Standard H-RDiagram

mV

B-V

Color-MagnitudeDiagram


32/39

32

Color-Magnitude Diagrams

Useful for star clusters Can substitute mV for MV since you know all the stars are the

same distance away.

Star Clusters

Open (galactic)

Globular


33/39

33

Structure of

the Milky Way


34/39

34

Open Clusters

Irregular shape Few tens to few hundred stars

In the plane of the galaxy

Young stars


35/39

35

Open clusters

M16

M45

M37

o or agn u e agram


36/39

36

o or- agn u e agramM45

0

2

4

6

8

10

12

14

16

18

-0.5 0 0.5 1 1.5 2B-V

mV


37/39

37

Globular Clusters

Spherical in shape Hundreds of thousands of stars

Halo distribution about galactic nucleus

Old stars


38/39

38

Globular Clusters

M5 M3

SFA Observatory

o or agn u e agram


39/39

39

o or- agn u e agramM3

Observational Astronomy 4

Documents