En anglais

Reading starlight line by line

Science · Astronomy & Astrophysics · ages 20-21

Name ______________________ Date ____________

What ordering does the main spectral sequence O, B, A, F, G, K, M describe?
- From the youngest stars to the oldest stars
- From the nearest stars to the farthest stars
- From the hottest stars to the coolest stars
After accounting for dust reddening and the instrument's response, what does a star's thermal continuum shape help estimate?
- Its surface temperature
- Its exact hydrogen abundance
- Its distance from the observer
Ionised hydrogen can produce Balmer absorption through bound electron transitions without first capturing an electron.
Circle one: True False
Which set lists intrinsic mechanisms that can broaden a stellar spectral line?
- Thermal motion, collisions, and rotation
- Uniform recession, distance, and sky position
- Uniform approach, distance, and exposure duration
Why does visible Balmer absorption reach its greatest strength near A-star temperatures?
- These stars contain far more hydrogen than every other class
- Enough neutral hydrogen has electrons in the n = 2 level
- All hydrogen nuclei absorb visible photons without electrons
An ideal symmetric absorption dip keeps its centre at 510 nm. Using the same width definition, its width increases from 1 nm to 3 nm. Which description fits?
- Its centre shifted red while its width stayed fixed
- Its centre shifted blue while its width became smaller
- Its width increased while its centre stayed fixed
An observer uses only a star's red continuum to claim that it has little hydrogen. Which method is needed to estimate its hydrogen abundance?
- Model hydrogen lines using temperature and pressure
- Treat continuum redness as a direct hydrogen count
- Treat continuum brightness as a direct hydrogen count
A line's laboratory wavelength is 600 nm. Its centre in the stellar spectrum is 601 nm, with unchanged width. Assume only radial Doppler motion causes the shift. What follows?
- The star approaches; fixed width proves each broadening cause is unchanged
- The star recedes; fixed width does not identify individual broadening causes
- The star rotates faster; fixed width proves its atmospheric pressure has fallen

Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading starlight line by line W1-mt_4FBh5G7Gdc-s1

From the hottest stars to the coolest stars · O is the hot end and M the cool end of the main stellar spectral sequence. The sequence orders surface temperature, not age or distance.
Its surface temperature · The distribution of thermal emission across wavelengths depends on temperature. The continuum shape therefore helps estimate surface temperature.
False · Ionised hydrogen has no bound electron. It must capture an electron before a bound-level Balmer absorption transition is possible.
Thermal motion, collisions, and rotation · Thermal motion and rotation spread the Doppler shifts contributing to a line. Collisions contribute pressure broadening. Instrumental width must also be considered in measurements.
Enough neutral hydrogen has electrons in the n = 2 level · Balmer absorption starts at n = 2 in neutral hydrogen. Cooler atmospheres have little excitation to that level; hotter atmospheres ionise more hydrogen.
Its width increased while its centre stayed fixed · Both central wavelengths are 510 nm, so the centre has not shifted. The width increases from 1 nm to 3 nm, which is broadening.
Model hydrogen lines using temperature and pressure · Continuum redness does not count hydrogen. Hydrogen line strengths need interpretation with atmospheric temperature and pressure, which affect excitation and ionisation.
The star recedes; fixed width does not identify individual broadening causes · 601 nm is longer than 600 nm, indicating recession under the stated assumption. Equal total width does not establish which individual broadening effects are present or unchanged.