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Questions tagged [astrophysics]

Questions involving the physics of the universe, especially the nature of astronomical objects, energy fields, and/or regions, rather than their positions or motions in space.

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35
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2answers
35k views

How does neutron star collapse into black hole?

We know the spectacular explosions of supernovae, that when heavy enough, form black holes. The explosive emission of both electromagnetic radiation and massive amounts of matter is clearly observable ...
32
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3answers
9k views

What happens to the 99.9 % of the sun-rays that do not fall on any planets or any other celestial body?

I assume that about 99.9% of the sun-rays that do not fall on any planet or any other celestial body keep on traveling farther and farther unto infinity. Apparently such rays get lost. Keeping in mind ...
29
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5answers
5k views

Quantum Mechanics after the detection of Gravitational Waves

Of course everyone knows by now of the detection of Gravitational waves But, since General Relativity and Quantum Mechanics don't get along, can we say now that this detection proves that Quantum ...
22
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4answers
11k views

Why can't we feel the Earth's revolution?

I googled it and checked a few Q&A and there's only things about "Earth's rotation". But why can't we feel the revolution? They say we can't feel the rotation because the Earth spins at a ...
22
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3answers
7k views

What would happen to a polished marble statue left in space for a million years?

Consider it doesn't collide with any other objects. Would it be preserved perfectly in the vacuum or would its surface be damaged by anything like UV rays, radiation, gas, space dust, etc?
17
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3answers
653 views

How does the evolution of a solar system not break the second law of thermodynamics?

Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck. As I understand it, the solar system evolved from a ...
16
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5answers
8k views

If all stars rotate, why was there a theory developed that requires non-rotating stars?

According to Penrose's research, a non-rotating star would end up, after gravitational collapse, as a perfectly spherical black hole. However, every star in the universe has some kind of angular ...
16
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0answers
505 views

Is it “weird” that all disc galaxies rotate once every billion years?

In a recent paper (Cosmic clocks: A Tight Radius - Velocity Relationship for HI-Selected Galaxies by Meurer, et al.), it was noted in the conclusion that: [This] implies a constant orbital time of ∼...
13
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1answer
1k views

What causes the dimensions of a star increase when its hydrogen fuel is exhausted?

What causes the dimensions of a star increase when its hydrogen fuel is exhausted? For example, the Sun is expected to increase its radius 250 times. What causes this if its temperature is expected to ...
13
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2answers
648 views

Open problems in astronomy that an amateur (with a PhD in some other field) would have a chance of solving?

What are some open problems in astronomy that an amateur would have a chance of solving? Suppose the amateur has a PhD in some other field, owns a basic telescope, a set of filters, diffraction ...
12
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2answers
1k views

Is it possible that a ultra-large portion of the space we live in is already inside a black hole? How could we refute this?

Trying to wrap my head around some concepts involving the very large gravitational fields of black holes, and what the gravitational gradients look like on a cosmic scale. I'm familiar with the Great ...
11
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3answers
1k views

How does a gravity slingshot actually work?

From what I know of elliptical orbits, an object speeds up near the periapsis and slows down at the apoapsis, much like we learned in high school physics how a sphere would roll down and back up a ...
10
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2answers
447 views

Would a tablespoon of a neutron star remain intact?

I've heard people say that a tablespoon of neutron star would weigh over a billion tons. If we ever could take a tablespoon of one would it still remain intact with the same density?
10
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1answer
727 views

Timescale of ignition of a protostar?

What is the timescale of start of nuclear fusion as T Tauri type star transforms into a Main Sequence star? Wikipedia article on T Tauri type stars mentions: Their central temperatures are too low ...
9
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2answers
2k views

Why do certain moons have their rotational period equal to their orbital period?

Our moon and Saturn's moon, Titan, have this feature. Because of this we only observe one side (hemispere) of our moon. Why is this? What is the Newtonian or astrophysics that would explain this? ...
9
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1answer
171 views

Period coincidence of Planet Nine with furthest Kuiper belt objects

A lead scientist of the Planet Nine theory said that "The period with which it goes around the sun is a rational multiple of the periods of all the furthest Kuiper Belt objects" To what degree of ...
8
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2answers
148 views

Why do stars born in cluster finally drive away from each other

In other words, if a group of stars born within the same nebula, e.g. Pleiads, are initially gravitationally bound - what causes them to finally get separated and proceed individually? Is it the ...
8
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1answer
112 views

How can the life time of a multiple star system, such as for example the trinary system PSR J0337+1715 be derived?

As for example explained at the beginning of this blog post, the trinary system consists of a millisecond pulsar ($1.438$ times the mass of the sun) orbited by two white dwarfs. One of the white ...
8
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1answer
157 views

Estimates for “undetectable” planets in extra-solar systems

As far as I've been able to determine, it appears there are some significant limitations on our ability to detect exo-planets that are below certain sizes or beyond certain distances from their stars....
7
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2answers
577 views

Can I see Harvest moon in India?

I just saw articles which say the Harvest moon will be visible in the US. Apart from the US, in which other country can people see it? Can I see it from India?
7
votes
2answers
353 views

Why do we use FITS format for scientific images especially in astronomy? How is it different from formats such as JPEG, PNG etc?

I am new to astronomy and I found that the telescopic dataset can be stored in the FITS format instead of JPEG. Why do we have a special image format just for scientific images?
7
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2answers
198 views

What was the length of the solar day 73 million years ago?

How could this be estimated, or how accurately could it be extrapolated from a present-day measurement of the Earth's spin-down rate?
7
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2answers
583 views

Are there real uses of abstract algebra in astrophysics?

It is known that a physicist must have, at least in general, a good level in mathematics in order to research and understand other people's works. Linear algebra is a good example, as ODEs and ...
7
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1answer
102 views

Gas giants and seismology

A long time ago I read a very interesting way of studying the sun's internal structure via sound waves. This new field of study, probability already known here, is called helioseismology. How much ...
6
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1answer
176 views

Is any consensus forming on the solution to the “Lithium Problem”?

The "Lithium Problem" relates to the fact very-low-metallicity stars appear to have a Li/H ratio approximately one third of what would be expected. The ratio should be the same as the prediction from ...
6
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1answer
234 views

What do the acronyms in “AT 2017 gfo” mean?

About 11 hours after the detection of GW170817 by Ligo/Virgo and the quickly succeeding detected GRB170817, a transient afterglow in the optical-nIR frequency appeared which showed many signs of r-...
6
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2answers
3k views

Our universe the surface of a 4-dimensional sphere?

The cosmic microwave background that we observe uniformly around us is usually explained by assuming that our universe is the surface of a four dimensional sphere. That way the uniformity makes sense ...
6
votes
1answer
119 views

What more could be learned from a rare astronomical event if we knew precisely when it would occur?

This is actually related to a question I recently asked on Worldbuilding, but seemed more appropriately asked here. To keep this from being too broad in scope, let's assume that someone figured out ...
6
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2answers
223 views

Assuming a sufficient amount of mass above the density threshold, does the actual concentration of the mass matter in creating a black hole?

I've heard that the actual density of a black hole (within the volume of the event horizon) is pretty low - does that mean that any sufficiently large volume of mass over that density is also a black ...
6
votes
1answer
1k views

What is the reason the Jovian planets are in descending order of size?

Is there any scientific reason for the jovian planets to be in descending order of size or is it purely coincidental? In terms of radii it goes Jupiter, Saturn, Uranus and Neptune.
6
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2answers
439 views

Weight of a celestial body

How do astronomers find out the correct weight of a planet even though there isn't any direct means to weigh them? What technique do they use? Just curious!
6
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3answers
171 views

Universe isn't infinite theory

Forgive me if any of my terminology is off I am no physicist but I have wondered about this question for a while. I have read in various books that if the universe was infinite, and uniformly created ...
5
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1answer
662 views

How does gravity affect pressure under water?

How different would being under water in Europa differ from being under water on Earth? Would it be possible to dive deeper on Europa with a submarine than on Earth and by how much?
5
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3answers
365 views

Why do neutron stars collide instead of just revolving around each other like planets revolve around the Sun?

We know about events like two neutron star colliding and resulting in a black hole, also collision of black holes, and collision of galaxies. But we never see a satellite such as a moon colliding with ...
5
votes
1answer
219 views

How to know that the 21cm Hydrogen line is the actual emission and not any other redshifted line?

Is it possible that we mistake some other wavelength emission to be 21 cm because of redshift?
5
votes
1answer
146 views

Delay between neutrino pulse and visible flash of supernova

If you were orbiting a star undergoing a core-collapse supernova, what would be the delay between the neutrino pulse from the collapse and the first visible effects on the surface? Basically, I'm ...
5
votes
2answers
252 views

Is there a lower limit for the altitude of orbiting objects?

As I understand it, one object can orbit another at a variety of altitudes, and the stability of the orbit is determined by (among other things) the speed of the orbiting object. Go too slowly and ...
5
votes
3answers
130 views

Can't pulsars and stars be used for gravitational wave measurement?

Suppose that we collected photons from a distant star, and they arrive regularly at 15 photons every microsecond onto a CCD, when a gravity wave bends space time, wouldn't the regular 15 photons graph ...
5
votes
2answers
1k views

Observational Evidence for the Existence of Black Holes

What is the direct observational evidence that black holes exist? I have read that black holes have been detected through X-ray emission. What is some other observational evidence for black holes?
5
votes
4answers
311 views

How fast is the universe expanding?

I've hear several theories stating that the universe is expanding faster than the speed of light, others claim that the universe expands faster the further away you measure it. Which of this is ...
5
votes
1answer
133 views

Superscript-subscript uncertainty notation

While reading publications in astrophysics I've encountered this kind of notation (fictitious example): $$M = 6.1^{+0.6}_{-0.1} \times 10^{30} \textrm{ kg}.$$ Several examples are here, p. 2. I ...
5
votes
1answer
124 views

A question about calculating the radius of a comet

A Comet is orbiting around the sun with period of $T= 5.5$ years. The comet has radius $R$. After 5.5 years (A period) we understand that its radius has decreased about $20cm$. How can we calculate ...
5
votes
1answer
206 views

Gravitational pull needed to keep a gas in atmosphere

How can you determine the gravitational force needed to keep a particular gas in the atmosphere of a planet (for example, carbon dioxide (CO2))? I came across the following formula $\left(\frac{8RT}{...
5
votes
1answer
913 views

What is the actual black hole merger speed?

I have seen models of 2 black hole mergers. https://www.theverge.com/2017/11/16/16665132/gravitational-waves-ligo-virgo-black-holes-merger-multi-messenger-astronomy When we see a space ship appear ...
5
votes
1answer
87 views

GW from merging of neutron stars and black holes

If it had been detected gravitational waves from merging of black holes (BH), and neutron stars (NS), why there are no detection of black holes merging with neutron stars? Why a BH-NS merging should ...
5
votes
1answer
431 views

Is our universe stable or metastable?

I've been reading and thinking about this subject for some time, but I can't seem to find a clear answer. After reading Q14: Aspects of the Higgs boson suggest that our Universe is only “...
5
votes
2answers
225 views

K-Pg asteroid impact

Is it possible to determine what daytime the Chicxulub impactor striked the Earth 66 mya? Was it day or night, what was the current phase of the Moon etc.? Can sediments, rhytmites or something else ...
5
votes
1answer
126 views

What happens to the Gas Pressure when working out the Eddington Luminosity?

I am looking at how the Eddington Luminosity is derived and I do no understand why we only care about $P_{rad}$? When working out $L_{Edd}$ you take the ratio between $dP_{rad}/dr$ and $dP_{...
5
votes
2answers
708 views

What uncertainty does an error bar signify in astronomy?

When an astronomer talks about her/his topic and shows an X/Y-plot with error bars. What should one assume that those error bars represent? 1 standard deviation? Or 2? Or some specific significance ...
4
votes
2answers
857 views

Circular formation around the moon

I have seen a large circular formation of cloud around the full moon sometimes. Have anyone seen the same? What is the reason behind this formation? Due to lunar attraction? Most of the sky is empty ...