Spectral Classification

Question 1

What is a good graph to plot to observe the spectra of something?

Answer 1

It’s flux against it’s wavelength.

Question 2

What do absorption lines look like on a flux-wavelength graph?

Answer 2

Big dips in the curve.

Question 3

Who made the two big discoveries about absorption lines and what were they?

Answer 3

• 1802 Wollaston discovers absorption lines

- 1811: Fraunhofer studies these absorption lines and labels them A, B, C, ….

Question 4

How did Fraunhofer discover the solar spectrum?

Answer 4

• He allowed sunlight to pass through narrow opening in shuttered room
• Compared the sunlight to the lamplight to see if similar bright stripe was to be seen
• Instead found with the telescope countless strong and weak vertical lines

Question 5

How can we get a graph of intensity vs wavelength without plotting it?

Answer 5

Scan a photographic plate.

Question 6

Who made the third big discovery about stellar spectra and when?

Answer 6

1859-Kirchoff & Bunsen discover absorption lines in the lab.

Question 7

What are the three types of spectra?

Answer 7

Continuous Spectrum, Emission spectrum and Absorption spectrum

Question 8

Whats the difference between the three types of spectra?

Answer 8

Continuous has no lines, emission has no continuum only bright lines, and absorption has a continuum with dark absorption lines.

Question 9

How are emission lines formed? How are absorption lines formed?

Answer 9

Emission lines are formed when light passes through a hot gas. Absorption lines are formed when hot light passes through cold gas.

Question 10

How do absorption lines form in stars?

Answer 10

Hot gas feeds energy into cooler layers at the surface, and then radiates.

Question 11

What was the first element discovered in the Sun?

Answer 11

Helium.

Question 12

How was spectra recorded from 1880-1970’s?

Answer 12

Recorded on photographic plates, so lines appeared as bright lines.

Question 13

What did Fleming do in 1890?

Answer 13

Sorted spectra of ~10000 stars by decreasing strength of the hydrogen Balmer lines, in the sequence A,B,C,…

Question 14

What did Cannon do in 1901?

Answer 14

Re-ordered the classification, and added decimal subdivisions (A0-A9), classified 225300 stars.

Question 15

What is the Harvard spectral classification?

Answer 15

Categorises stars based on their temperature (goes from hottest to coolest).

Question 16

What is the Rydberg formula?

Answer 16

1/λ = RZ^2 *(1/n’^2 - 1/n^2)

Question 17

What is the order of letters in the Harvard spectral classification? What happens in each section?

Answer 17

• O: hot stars, strong He II
• B: He I strong
• A: Strongest balmer lines
• F: Strong Ca II, ionised and neutral metals
• G: Strong Ca II, numerous neutral metals
• K: Ca II getting weaker, numerous neutral metals
• M: Very strong molecular bands

Question 18

What does it mean when there is an I next to an elements symbol?

Answer 18

I means neutral, II means first ionisation etc

Question 19

What does the graph of log(F) against wavelength look for each spectral type?

Answer 19

Lower and lower intensity as log(F) gets lower, and temperature.

Question 20

What Teff do each colours have?

Answer 20

blue-white = 30000K, white = 10000K, yellow = 5000K, red = 3000K and below

Question 21

What are the two big deviations real spectra have from black bodies?

Answer 21

Absorption lines and balmer limit.

Question 22

What are the 7 luminosity classes?

Answer 22

• I=supergiants
• II=bright giants
• III=giants
• IV=subgiants
• V=main-sequence
• VI=subdwarfs
• D=white dwarfs

Question 23

What did Cecilia Payne discover in 1925?

Answer 23

Different line strengths are not due to wildly varying abundances, to the temperature dependence of the Boltzmann and Saha equations.