# Anyone up for a Astrophysics Marathon? (1 Viewer)

##### Member
Just wondering, if we can just focus on this (as it is my weakest module =S), and how many people actually do this module on these forums.

Question: Explain how two other characteristics or properties of a star can be deduced by observing the light received from the star.

#### teeah

##### Member
I do astro; doesn't seem like many people do though :\

I think this question actually came up in my trials. I talked about surface temperature (i.e. blue shifted lines = higher surface temperature) and translational velocity (looking at the Doppler Shift to determine it's movement - i.e. when blue shifted, star is approaching observer)

##### Member
I do astro; doesn't seem like many people do though :\

I think this question actually came up in my trials. I talked about surface temperature (i.e. blue shifted lines = higher surface temperature) and translational velocity (looking at the Doppler Shift to determine it's movement - i.e. when blue shifted, star is approaching observer)
LOL, once you finish with the question you answered you're meant to also post up a question.

#### J-Wang

##### Member
In relation to the first question, I would talk about how the colour can help determine its surface temperature, hence alowing it to be plotted on the HR diagram where M can be found and hence its distance. Also, discuss how this plotting can calculate its mass and hence elements present withing the star etc through absorption lines

Another one that always comes up in astrophysics is the death cycle of a star. Anyone wish to expand?

#### teeah

##### Member
haha yeah, just remembered: Identify THREE advances in measurement technologies, and describe how they
have improved our understanding of celestial objects.

#### barbernator

##### Active Member
I do astro; doesn't seem like many people do though :\

I think this question actually came up in my trials. I talked about surface temperature (i.e. blue shifted lines = higher surface temperature) and translational velocity (looking at the Doppler Shift to determine it's movement - i.e. when blue shifted, star is approaching observer)

- You said "blue-shifted lines" denotes a higher surface temperature, yet it is higher intensity of blue light received, not the shifting of spectral lines that denotes a hotter surface temperature. This can be explained as a black body radiator emits a higher intensity of shorter wavelengths as temperature increases.
- I also may not talk about the translational velocity because it may not be directly attacking the "properties of a star" itself.

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#### habitres

##### Member
Yea I'm keen for this. Answer is what teeah said and also (i just got a new keyboard, soo soo good) and yea, also that by examining spectral lines you can determine the density of the star due to the fact that less dense starts have longer photon travel before collisions in the atmosphere.

What is the significance of the period-luminosity relationship of Cepheid variables?

#### barbernator

##### Active Member
In relation to the first question, I would talk about how the colour can help determine its surface temperature, hence alowing it to be plotted on the HR diagram where M can be found and hence its distance. Also, discuss how this plotting can calculate its mass and hence elements present withing the star etc through absorption lines

Another one that always comes up in astrophysics is the death cycle of a star. Anyone wish to expand?
and wanga, I wouldn't talk about the highlighted because the first isnt a characteristic of the star itself, and the second involves the HR diagram which might be going a little too far away from the initial question of just the light from the star.

but yep deffs talk about spectral lines = elements.

#### someth1ng

##### Retired Nov '14
Explain how two other characteristics or properties of a star can be deduced by observing the light received from the star.
I'm just going to be brief on what I would write.

1. The most intense wavelength of light can be used to determine surface area of the star as according to Wien's Law which states that the product of the surface temperature and most intense wavelength is a constant for all stars.
2. The spectral lines of a star can be used to determine the atoms, molecules and/or ions present in a star.
3. Other things could include thickness of spectral lines provide information on the density of a star's atmosphere (thicker lines are produced by denser atmospheres) as well as rotational velocity of a star. Furthermore, red or blue shifting of spectral lines can be used to determine translational velocity of a star on the observer's line of sight which can also be used to estimate distance.

Identify THREE advances in measurement technologies, and describe how they have improved our understanding of celestial objects.
I hate these questions - are telescopes counted as "measurement technologies"?

If so, I'd talk about active optics, adaptive optics and interferometry which isn't too hard.

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#### jenslekman

##### Member
and wanga, I wouldn't talk about the highlighted because the first isnt a characteristic of the star itself, and the second involves the HR diagram which might be going a little too far away from the initial question of just the light from the star.

but yep deffs talk about spectral lines = elements.
lol - how are people approaching the seven markers such as:

Astronomers employ a range of instruments and techniques to observe celestial
objects.

Assess the impact of technological advances on our understanding of the
cosmos.

##### Member
What is the significance of the period-luminosity relationship of Cepheid variables?
Anyone?

#### someth1ng

##### Retired Nov '14
lol - how are people approaching the seven markers such as:

Astronomers employ a range of instruments and techniques to observe celestial
objects.

Assess the impact of technological advances on our understanding of the
cosmos.
Talk about 2 of each such as:
Telescopes: adaptive optics, active optics and interferometry
Spectrographs and spectroscopes: measuring spectra of stars
Trigonometric parallax
Spectroscopic parallax OR cepheids

##### Member
The analysis of electromagnetic radiation is widely used by astronomers. Contrast emission and absorption spectra in terms of how they are produced.

#### habitres

##### Member
The analysis of electromagnetic radiation is widely used by astronomers. Contrast emission and absorption spectra in terms of how they are produced.
Emission spectra - produced by interstellar gasses and dust that are heated by surrounding stars/excited by external means. The electrons absorb energy, and in doing so jump up an energy level/sub shell. (chemistry of art ftw) From this higher energy state, the electron(s) drop back down to their previous state in a variety of different paths etc, each sequential 'drop' releasing a different wavelength of light photon that is emitted outwards. (Eg. hydrogen emission spectra).
When observing the spectra, it would be characterised by coloured lines (the different wavelengths) against a black background.

Absorption spectra - occurs in the outer layers of stars. Photons released by the star are absorbed by the outer gas layers and re-emitted, usually in a different direction from their initial travel. When observing the spectra of this, it would be characterised by black lines against a full/rainbow like spectrum.

Ill put another questions since no one answer my previous one.

What are the values of the two 'limit's' in regards to stars mass/size, and how do they relate to alter the sequence of events occurring at the death of a star?

#### someth1ng

##### Retired Nov '14
Emission spectra - produced by interstellar gasses and dust that are heated by surrounding stars/excited by external means. The electrons absorb energy, and in doing so jump up an energy level/sub shell. (chemistry of art ftw) From this higher energy state, the electron(s) drop back down to their previous state in a variety of different paths etc, each sequential 'drop' releasing a different wavelength of light photon that is emitted outwards. (Eg. hydrogen emission spectra).
When observing the spectra, it would be characterised by coloured lines (the different wavelengths) against a black background.

Absorption spectra - occurs in the outer layers of stars. Photons released by the star are absorbed by the outer gas layers and re-emitted, usually in a different direction from their initial travel. When observing the spectra of this, it would be characterised by black lines against a full/rainbow like spectrum.

Ill put another questions since no one answer my previous one.

What are the values of the two 'limit's' in regards to stars mass/size, and how do they relate to alter the sequence of events occurring at the death of a star?
That's quite an iffy response, to be honest with you. You really only gave EXAMPLES of how those spectra are formed but you didn't specifically tell HOW those spectra are formed.

By two limits, I'm assuming it's the Chandrasekhar limit and Tolman–Oppenheimer–Volkoff limit (TOV limit).

- The limits refer to the maximum mass of the core of a star during star death for a certain composition of be stably maintained.
- There are two main "limits", the Chandrasekhar limit and the Tolman–Oppenheimer–Volkoff limit (TOV limit).
- The Chandrasekhar limit is the maximum stable mass of a white dwarf.
-->Any core mass above approximately 1.4Mo will cause gravity to overcome electron degeneracy pressure, forcing electrons into the nuclei of atoms to interact with protons to form neutrons.
- The Tolman-Oppenheimer-Volkoff limit (TOV limit) is the maximum stable mass of a neutron star.
--> Any mass above this "limit" of approximately 3.0Mo will cause gravity to overcome neutron degeneracy pressure and resulting in the production of, usually, a black hole.

Question said:
Assess the problems of ground-based astronomy.

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#### shongaponga

##### Member
What is the significance of the period-luminosity relationship of Cepheid variables?

Cepheids are intrinsic periodic variables. The period-luminosity relationship of a Cepheid Variable star allows the distance to a Cepheid to be calculated. By observing the star through spectral analysis over time, the type of Cepheid can be determined and a characteristic light curve established(Type I Cepheids are more luminous than Type II Cepheids). From this light curve, the period of the star can be determined. The period-luminosity relationship states that Cepheids with longer periods of oscillation are more luminous, hence by using this relationship the average absolute magnitude(M) can be calculated. From photometric observation, the apparent magnitude(m) of the star can be found and hence via application of the distance-modulus formula the distance to the star can be calculated.

Q. Describe the physical processes that proceed nuclear fusion reactions in a newly formed star.

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#### kiinto

##### Member
Q. Describe the physical processes that proceed nuclear fusion reactions in a newly formed star.
- Gravity condenses hydrogen gas into a ball.
- Gravitational pressure is converted to thermal energy in the core of this ball.
- When thermal pressure outwards equals gravitational pressure inward, the star lights up and begins to fuse hydrogen.

By two limits, I'm assuming it's the Chandrasekhar limit and Tolman–Oppenheimer–Volkoff limit (TOV limit).

- The limits refer to the maximum mass of the core of a star during star death for a certain composition of be stably maintained.
- There are two main "limits", the Chandrasekhar limit and the Tolman–Oppenheimer–Volkoff limit (TOV limit).
- The Chandrasekhar limit is the maximum stable mass of a white dwarf.
-->Any core mass above approximately 1.4Mo will cause gravity to overcome electron degeneracy pressure, forcing electrons into the nuclei of atoms to interact with protons to form neutrons.
- The Tolman-Oppenheimer-Volkoff limit (TOV limit) is the maximum stable mass of a neutron star.
--> Any mass above this "limit" of approximately 3.0Mo will cause gravity to overcome neutron degeneracy pressure and resulting in the production of, usually, a black hole.
Is this in the syllabus?

Assess the impacts of the improvement in photometric technologies on the understanding of our universe

Elements up to Iron (i.e. #26) are formed within stars. Explain how heavier elements came into existence

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#### habitres

##### Member
Is this in the syllabus?
Yes and No. When describing lives of stars, you need to determine the SM differences between white dwarfs, neutron stars and black holes, but don't need to name the limit. More so for useful information! (Now you know WHY the limits are what they are!)

#### nirukk

##### Member
Q. Describe the physical processes that proceed nuclear fusion reactions in a newly formed star.
When a newly formed protostar acquires an equilibrium such that its gravitational collapse is balanced by an outward pressure from the heat by its core. The protostar then accretes more material from the cloud surrounding it which then blows off the rest to reveal itself which was previously hidden from our view. When the star is hot enough, it beings fusion of hydrogen in its core.

#### kiinto

##### Member
Explain why a white dwarf does not collapse further. (2mks)