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Thread: Astronomy Questions

  1. #1 Astronomy Questions 
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    Hi All - Here are some review questions I am having trouble with, can you help me?

    1) Why are hydrogen lines not seen in the absorption spectra of some stars?

    2) A star of low surface temperature surrounded by a higher temperature cloud of gas would show a...

    3) Visible absorption lines of elements in a star's spectrum occur because...

    5) If you see a continuous spectrum with a spectroscope, then you know that the source of the spectrum is...

    6) The pattern and strength of lines in a spectrum of a star allows you to determine the...

    7) What is the relation between the apparent magnitude of a star and its color?

    8) If a star has a parallax of 0.1 seconds of arc and an apparent brightness of 0.7, what is the absolute magnitude?

    9) When light is doppler shifted toward the blue end of the spectrum...

    10) In order to evaluate Hubble's constant for distant galaxies, what quantities must you measure?

    11) We can use Hubble's Law and the Hubble constant to estimate the age of the universe. In doing so, the larger the value of the Hubble Constant, then the age of the universe is.............

    12) Which observed property of the 3K background radiation implies most strongly that it has a cosmic origin?



    Thank you!


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  3. #2 Re: Astronomy Questions 
    Reptile Dysfunction drowsy turtle's Avatar
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    Quote Originally Posted by lucky7
    Hi All - Here are some review questions I am having trouble with, can you help me?

    1) Why are hydrogen lines not seen in the absorption spectra of some stars?
    Because the hydrogen inside a star is hot enough that it also emits the relevent frequencies. When the emission and absorbtion spectra of hydrogen are combined, you get a full spectrum. (educated guess, no sources)

    Quote Originally Posted by lucky7
    2) A star of low surface temperature surrounded by a higher temperature cloud of gas would show a...
    Planetary nebula/red giant.

    Quote Originally Posted by lucky7
    3) Visible absorption lines of elements in a star's spectrum occur because...
    Because the elements are absorbing the light?

    No #4?

    Quote Originally Posted by lucky7
    5) If you see a continuous spectrum with a spectroscope, then you know that the source of the spectrum is...
    Not absorbing or emitting light to any great extent?

    Quote Originally Posted by lucky7
    6) The pattern and strength of lines in a spectrum of a star allows you to determine the...
    relative percentages of elements present.

    Quote Originally Posted by lucky7
    7) What is the relation between the apparent magnitude of a star and its color?
    Bear in mind that higher frequency radiation has greater energy, and you should be able to reach an answer.

    Quote Originally Posted by lucky7
    9) When light is doppler shifted toward the blue end of the spectrum...
    Moving towards.


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  4. #3 Re: Astronomy Questions 
    Moderator Moderator Dishmaster's Avatar
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    Quote Originally Posted by lucky7
    Hi All - Here are some review questions I am having trouble with, can you help me?

    1) Why are hydrogen lines not seen in the absorption spectra of some stars?
    These lines are formed in the "atmosphere" of cooler stars, where the hydrogen is neutral. In hotter stars, more atoms tend to be ionised. Hence, less neutral atoms to produce the spectrum.

    2) A star of low surface temperature surrounded by a higher temperature cloud of gas would show a...
    Depending on the size and density of the cloud, a mixture of absorption and emission lines, where the emission lines have a different line width than the absorption lines.

    3) Visible absorption lines of elements in a star's spectrum occur because...
    See above. Cool layer of atomic gas or gas with low degree of ionisation in the stellar atmosphere.

    5) If you see a continuous spectrum with a spectroscope, then you know that the source of the spectrum is...
    ... emitting photons. It depends on the slope/shape of the continuous spectrum, what the source might be.

    6) The pattern and strength of lines in a spectrum of a star allows you to determine the...
    temperature, chemical composition; the width of the lines allows you to additionally determine the size, age, pressure, density.

    7) What is the relation between the apparent magnitude of a star and its color?
    None. The apparent magnitude is a product of intrinsic brightness, distance and extinction by interstellar matter. However, the relation of apparent magnitudes measured at different wavelengths, which is called colour in astronomy, is almost as informative as a spectrum. If you ignore extinction, the colour is a measure of the surface temperature.

    8) If a star has a parallax of 0.1 seconds of arc and an apparent brightness of 0.7, what is the absolute magnitude?
    I leave this simple measurement to you. You will need the distance modulus and the definition of a parsec for this.

    9) When light is doppler shifted toward the blue end of the spectrum...
    ... the source is moving towards you.

    10) In order to evaluate Hubble's constant for distant galaxies, what quantities must you measure?
    This is difficult. You would need an independent distance indicator, because the Hubble constant is the slope of the relation between distance and redshift.

    11) We can use Hubble's Law and the Hubble constant to estimate the age of the universe. In doing so, the larger the value of the Hubble Constant, then the age of the universe is.............
    Oh come on. That's easy. Just expand the units of the Hubble constant (km/s per Mpc), and you'll see.

    12) Which observed property of the 3K background radiation implies most strongly that it has a cosmic origin?
    Uniformity
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  5. #4  
    WYSIWYG Moderator marnixR's Avatar
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    moved this thread to its proper location
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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