Astronomy — Analyzing Starlight

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Theory — Analyzing Starlight

Starlight carries a star’s vital statistics. Spread into a spectrum, it reveals temperature and composition; measured over time, its position reveals distance.

1. Spectral classification

Stars are sorted by temperature into the sequence O B A F G K M, from the hottest blue O stars (over 30,000 K) to the coolest red M stars (about 3,000 K). Our Sun is a G star at about 5,800 K. The pattern of absorption lines in the spectrum sets the class.

2. Luminosity and the Hertzsprung Russell diagram

A star’s luminosity is its total power output. Plotting luminosity against temperature gives the Hertzsprung Russell diagram. Most stars lie on the diagonal main sequence; cool but very bright stars are giants; hot but faint stars are white dwarfs. Where a star falls tells you what stage of life it is in.

3. Distance from parallax

As the Earth orbits the Sun, a nearby star appears to shift slightly against the distant background. Half of that annual shift is the parallax angle p. Distance follows directly:

Parallax distance d = 1 / p
d in parsecs, p in arcseconds
A parallax of 1 arcsecond means a distance of 1 parsec (3.26 light-years).

The smaller the parallax, the farther the star. This is the first rung of the cosmic distance ladder.

4. The interstellar medium

Between the stars lies thin gas and dust. Dust dims and reddens starlight passing through it, and glowing clouds of gas form emission nebulae. Accounting for this material is part of reading starlight correctly.

Apparatus

Reading starlight uses instruments to spread, measure, and compare light. In the simulation these are modelled, but the readings match what each instrument would give.

Stellar spectrum

A star’s spectrum with dark absorption lines whose pattern sets its spectral class.

Hertzsprung Russell chart

A plot of luminosity against temperature showing the main sequence, giants, and white dwarfs.

Parallax baseline

The Earth’s orbit provides the baseline for the small annual parallax shift of a nearby star.

Spectrograph

Disperses a star’s light into a spectrum for classification and Doppler measurement.

Photometer

Measures a star’s brightness precisely so its luminosity can be found.

Telescope

Collects the faint light of distant stars for spectroscopy and photometry.

Instructions

Work through both tabs. Calculate first by hand, then press the button to compare.

Part A — Distance from parallax

Choose a star and read its parallax angle p in arcseconds.
Compute the distance d = 1 / p in parsecs by hand.
Enter your distance and press Check; it compares within 3 percent.

Part B — Spectral class and the Hertzsprung Russell diagram

Read a star’s temperature and luminosity.
Predict its spectral class and where it sits on the diagram (main sequence, giant, or white dwarf), then press Check.

Simulation

Star

Parallax p (arcsec)

Your distance d (pc)

Temperature (K)

Luminosity (Suns)

Spectral class

Position

Team Questions

1. A star with a parallax of 0.5 arcsecond is at a distance of:

0.5 parsec 5 parsecs 2 parsecs 50 parsecs

2. The hottest stars in the spectral sequence are class:

O G M K

3. On the Hertzsprung Russell diagram, most stars lie along the:

White dwarf region Main sequence Giant branch only Bottom right corner

Example Report

Worked example: the distance to Sirius

Sirius has a measured parallax of p = 0.379 arcsec.

d = 1 / p = 1 / 0.379 ≈ 2.64 parsecs, which is about 2.64 × 3.26 ≈ 8.6 light-years.

Sirius is about 8.6 light-years away, so the calculated and accepted values agree. Measuring the parallax and inverting it to get the distance is the calculate-then-compare core of the lab.

Analyzing Starlight