Assignment #5– The H-R diagram – Finding out about stars.

In the early part of the 20th century Ejnar Hertzsprung and Henry Norris Russell worked at plotting the properties of stars. To their amazement, they discovered that stars do not fall randomly on these plots, but fall into distinct groupings when absolute magnitude or luminosity is graphed versus temperature/spectral type/colour. In fact it was found that 90% of stars fall along a distinct line we now refer to as the main sequence.

The main groupings on the HR diagram are referred as ‘luminosity classes’. They are as follows

Luminosity classes

Ia – most luminous supergiants

Ib – less luminous supergiants

II – luminous giants

III – normal giants

IV – subgiants

V - main sequence

Our sun for example is a G2V star. That means its spectral type is G2, and since it is on the main sequence it is also a V.

This diagram became a very important tool for stellar astronomers. The grouping of the stars was a key to the evolution of stars. It also allows us to deduce stellar characteristics based on where a star falls on the chart. If we examine a star’s spectra we can then figure out its age, size, mass, temperature and colour.

The different luminosity classes tell us where the star is in its lifespan. Also we have discovered more recently an additional class ‘white dwarfs’ that are at the end of a stars’ lifespan, and that are in the lower left hand corner of the HR diagram.

The following assignment allows students to create their own HR diagram by plotting some of the nearest and brightest stars on their own graph. The HR diagram for this exercise will plot the absolute magnitude versus spectral class.

Absolute magnitude is a measure of how bright a star actually is, irrelevant of how far it is. (Just as a 100W light bulb is certain brightness, but depending on your distance it can be very faint or very bright). The magnitudes often listed for observers, or on your star charts are visual magnitudes; this is how bright a star actually appears to the observer standing on Earth. For example our sun’s visual magnitude is around –26, while it’s absolute magnitude is only around 4. (Please note the magnitude scale is backward, the lower the number the brighter the star, this dates from Hipparchus, when 1 was a bright star and 9 a dim star)

Spectral type is a classification based on the appearance of a star’s spectra. There are seven main types, these are O, B, A, F, G, K, M. These types are subdivided into 0-9. The shape of the spectra of these objects is examined, and based on certain key features, including the Hydrogen lines, a spectral type is found.

The following chart gives the common name, absolute magnitude (y-axis) and the spectral type (x-axis) for stars for the HR diagram you will plot. A graph on which you will plot the stars will be handed out in class. Although you do not have to number all of the stars, it is suggested that you should number some in order to be able to answer the questions.

Once you have plotted the diagram you will answer questions about it, and the stars plotted on it.

Common Name / Abs. Mag. / Spectype / Common Name / Abs. Mag. / Spec type / Common Name / Abs. Mag. / Spec type
Sirius A / 1.45 / A1 / Bellatrix / -2.72 / B2 / a Centauri B / 5.70 / K1
Canopus / -5.53 / F0 / Miaplacidus / -0.99 / A2 / Barnard's Star / 13.24 / M4
Arcturus / -0.31 / K1 / Alnilam / -6.38 / B0 / Ross 154 / 13.00 / M3
Rigel Kentaurus / 4.34 / G2 / Alnair / -0.73 / B7 / e Eridani / 6.18 / K2
Vega / 0.58 / A0 / Alioth / -0.21 / A0 / Lacaille 9352 / 9.76 / M0
Capella / -0.48 / G5 / Kaus Australis / -1.44 / B9 / FI Virginis / 13.50 / M4
Rigel / -6.69 / B8 / Mirphak / -4.50 / F5 / 61 Cygni A / 7.49 / K5
Procyon A / 2.68 / F5 / Dubhe / -1.08 / K0 / Procyon / 2.68 / F5
Betelgeuse / -5.14 / M1 / Wezen / -6.87 / F8 / 61 Cygni B / 8.33 / K7
Achernar / -2.77 / B3 / Sargas / -2.75 / F1 / GX Andromedae / 10.33 / M1
Hadar / -5.42 / B1 / Avior / -4.58 / K3 / e Indi / 6.89 / K4
Altair / 2.20 / A7 / Menkalinan / -0.10 / A2 / t Ceti / 5.68 / G8
Acrux / -4.19 / B0 / Atria / -3.62 / K2 / YZ Ceti / 14.25 / M4
Aldebaran / -0.63 / K5 / Delta Velorum / -0.01 / A1 / Luyten's Star / 11.94 / M3
Spica / -3.55 / B1 / Alhena / -0.60 / A0 / Kapteyn's Star / 10.89 / M0
Antares / -5.28 / M1 / Peacock / -1.81 / B2 / AX Microscopii / 8.71 / M2
Pollux / 1.09 / K0 / Polaris / -3.64 / F7 / 70 Ophiuchi A / 5.50 / K0
Formalhaut / 1.74 / A3 / Mirzam / -3.95 / B1 / s Draconis / 5.87 / K0
Deneb / -8.73 / A2 / Alphard / -1.69 / K3 / h Cassiopei o / 4.59 / G0
Regulus / -0.52 / B7 / Hamal / 0.48 / K2 / van Maanen's Star / 14.15 / wd
Castor / 0.59 / A1 / 36 Ophiuchi C / 7.45 / K5 / Sirius B / 11.34 / wd
Gacrux / -0.56 / M3 / 37 Bootis / 5.41 / G8 / Procyon B / 12.98 / wd


Questions

1.  Plot the stars on the provided diagram. Hand diagram in.

2.  Label the following on the diagram - main sequence, red giants.

3.  To which group ( e.g. main sequence or red giants would be an example of a group) to the following stars belong? (this should include a colour as part of the answer)

  1. Sun
  2. Procyon A
  3. Atria
  4. YZ Ceti
  5. Luyten’s Star

4.  Find an example (other than stars in question 3) for the following categories.

  1. White dwarf
  2. Red main sequence star
  3. Red giant
  4. Yellow main sequence star
  5. K type star

5.  Why do you think Sirius and Procyon have both an ‘A’ and a ‘B’ star?

6.  Which is the hottest star on the diagram?

7.  Which is the coolest star on the diagram?


Name: Student Number: .

Answer Sheet - Exercise # 4

1 & 2 – Hand in HR diagram you have plotted

3.  Star groups

  1. Sun______
  1. Procyon A______
  1. Atria ______
  1. YZ Ceti______
  1. Luyten’s Star______

4.  Example of star type

  1. White dwarf______
  1. Red main sequence star______
  1. Red giant______
  1. Yellow main sequence star______
  1. K type star______

5.  Why ‘A’ and ‘B’?

6.  Hottest Star?

7.  Coolest star?