The science dealing with objects and phenomena located beyond Earth. In particular, this applies to observations and data. At its core, astronomy is the physically informed cataloging and classifying of the contents of the universe in order to better understand what is out there.

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Why can't Wetterich's “Universe without expansion” be tested compared to standard cosmological redshift?

I have read on several sites that this is a theory that cannot be tested. Why is this a theory that can not be tested? It seems to me that if the mass of particles is continually increasing, then the ...
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1answer
102 views

How did Kepler arrive at his laws?

How did Kepler arrive at his laws? If one already knows the distances to the planets (and the eccentricity of the orbits etc.) it is understandable how one might proceed to establish the second and ...
6
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1answer
351 views

Determine date of birth from “astrological” signs?

Suppose someone tells me what constellation the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn were in when they were born, and the information is astronomically accurate (ie, not astrological ...
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3answers
215 views

What happens to the electron companions of cosmic ray protons?

If primary cosmic rays are made mostly of protons, where are the electrons lost, and does this mean that the Earth is positively charged? Does the sun eject protons and electrons in equal number?
6
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1answer
88 views

How are stellar and galactic ages determined?

Being a 4th year undergraduate physicist, you'd think I'd know this! But it's never really taught in any detail, just vague mentions of metallicities and, in galactic case, redshift. So how exactly ...
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5answers
25k views
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1answer
149 views

Would a collapsing Universe have the density of water?

I understand that the typical density of the super massive black hole is close that of the water. It is also my understanding that this density is not true matter density because the volume used to ...
2
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0answers
61 views

Gamma ray bursts luminosity measurement

physicists use millisecond fluxes i.e.,fluxes calculated within a very short timescale to measure the actual 1 second flux for these grbs.But to do that they have to multiply the millisecond fluxes ...
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3answers
5k views

How fast will the sun become a red giant?

I've read many accounts of our sun's distant fate, but what I've never heard is on what time scale these events occur. For instance, when the sun runs out of hydrogen, I presume it doesn't just WHAM! ...
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41 views

Alma correlator for visible light and for space based telescopes?

I was shocked to see that one can simulate interferometry on a computer like the ALMA Correlator described by this video (posted on Wikipedia's article on ALMA) Is is possible to do the same in ...
2
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2answers
116 views

A thought about Quasars

If Quasars are "beams" of energy exiting a super-massive black hole, in order for them to get through the black-hole's event horizon, they'd have to be traveling faster than the speed of light. My ...
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1answer
57 views

Is it possible to clearly receive weak signal from space?

Imagine there is communication satellite on geostationary orbite. It transmits signal in a defocused beam, "illuminating" whole Earth hemisphere it's floating over. For example, signal is ...
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1answer
502 views

Radians to Rotate Earth to Match ECI Lat/Lon with ECEF Lat/Lon

I am attempting to model GPS Satellite positions on the globe for a set of ephemerides. I have a verified set of ECI ( http://en.wikipedia.org/wiki/Earth-centered_inertial ) XYZ Coordinates and a ...
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0answers
103 views

How fast do large asteroids usually travel?

I have just watched this video made by discovery channel, and you can hear the narrator at 0:51 saying that : "even though it is moving at 720 THOUSANDS kilometers per hour..." I stopped once I heard ...
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2answers
459 views

Magnitude of New Comet C/2012 S1 (ISON)

A new comet (magnitude 18.8) has been discovered beyond the orbit of Jupiter. Comet ISON will get within 0.012 AU of the Sun by the end of November 2013 and ~0.4 AU from of Earth early in January ...
19
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2answers
656 views

Optimal telescope size?

Consider a diffraction-limited telescope with unobstructed aperture $D$. Such a scope is capable of yielding an angular resolution $\alpha$ that scales as $\lambda/D$, with $\lambda$ denoting the ...
6
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1answer
209 views

Is it feasible to measure the energy of cosmic ray muons with a consumer Digital Single Lens Reflex camera?

I have read this article SIBBERNSEN, Kendra. Catching Cosmic Rays with a DSLR. Astronomy Education Review, 2010, 9: 010111. and it talks about estimating the muon cosmic ray flux by means of a DSLR ...
2
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2answers
787 views

What is meant by positive and negative gravity/energy/spactimecurvature?

I have recently come across some cosmological assertions (based on empirical data) about the universe being self contained in the sense that it is entirely capable of coming into existence from a ...
7
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1answer
148 views

“Cloud maps” of a nearby brown dwarf: what do they actually show?

This paper recently made headlines: A global cloud map of the nearest known brown dwarf. I. J. M. Crossfield et al. Nature 505, 654–656 (30 January 2014). In short, they claim they've been able ...
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3answers
57 views

Are our telescopes capable of taking actual images of brown dwarfs?

One nice result from NASA's WISE infrared survey of the sky is the discovery of particularly cool (as in 'not very warm') stars, now called 'Y dwarfs'. This was reported, for example, in NASA's ...
2
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1answer
204 views

When Venus is the brightest when viewed from Earth?

Nowadays Venus is very bright. I can spot it during broad daylight without problem. It's because it's near Earth and appear as a crescent. This made me think: as it's reaching the inferior ...
3
votes
1answer
505 views

Center of Mass of two planets

The formula for calculating the center of mass is $$ r_{center} = \frac{m_1 \cdot r_1 + m_2 \cdot r_2}{m_1+m_2} $$ Why can't I use it to calculate the barycentre of two planets? I understand how ...
27
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4answers
6k views

Why is a new moon not the same as a solar eclipse?

Forgive the elementary nature of this question: Because a new moon occurs when the moon is positioned between the earth and sun, doesn't this also mean that somewhere on the Earth, a solar eclipse ...
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0answers
69 views

How to measure distances to stars by means of spectroscopic parallaxes?

How to measure distances to stars by means of spectroscopic parallaxes on practice? What is the accuracy of measuring distances using this method compared with distances based on HIPPARCOS ...
16
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4answers
900 views

How would we see a near-lightspeed object emitting light?

Consider an object travelling near the speed of light relative to us (let it be a spaceship or a star), which is emitting light (consider it monochromatic resulting from a two level electronic ...
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1answer
154 views

How long would it take for a galaxy to collapse without dark matter?

I am trying to understand the effects of gravity in the Cosmos without complications of Dark Matter/Dark energy issues. So my question is, assuming that a galaxy (for example, the Milky Way) does not ...
3
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1answer
116 views

absorption and emission lines in Cassiopeia A hydrogen spectrum

we recently took a tour to a radio telescope and recorded some spectra, one of them being Cassiopeia A. Looking at the difference in on-source and off-source spectra, we find sharp absorption and ...
8
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1answer
2k views

What happened to Apollo's Saturn-third-stage rockets?

I read recently the original Apollo 11 press release and it mentions that the Saturn V's third stage (used for Trans-Lunar Injection) was deployed into a solar orbit of some kind: (Source: Press ...
4
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1answer
261 views

Velocity distribution in Plummer's models and others mass distributions

The Plummer's sphere is an model for the mass density in a globular cluster of stars. For an $N$-body simulation I have initialized the position of $N$ masses with a Monte-Carlo technique but cannot ...
3
votes
1answer
34 views

What compact central objects younger than the crab pulsar have been seen?

We see the crab pulsar, we don't see any compact remnant from Supernova 1987A. I can't find any others, but I believe they exist. Help?
25
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8answers
18k views

Why CAN we see the new moon at night?

I understand that the Moon's phases are determined by its position in orbit relative to the Sun. (See: Full Story on the Moon). The "shadow" is not cast by the Earth (a common misconception - this is ...
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1answer
48 views

Relation between red filter band and redshift?

I am interested in knowing what is the significance of red filter band in the study of redshift dependence of spatial orientation of galaxies?
4
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1answer
3k views

What's the difference between a red giant and a red supergiant?

I've seen both theoretical and observational definitions of stars. For example, an AGB star is a star where two sets of nuclear reactions (helium to carbon and hydrogen to helium) are taking place in ...
2
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2answers
273 views

The validity of the Longitude of Perihelion

As I understand it from Astronomical Algorithms, by Jean Meeus, the Longitude of Perihelion is a very common numeric value associated with planets, even used as one of the planetary orbital elements. ...
2
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1answer
116 views

Relation of fraction of binary stars with spectral class (mass)

What is relation of fraction of binary stars with spectral class (mass)? For example, how many binary stars are among O,B,A,F,G,K,M stars separately?
17
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4answers
1k views

Does the Moon's core still contain significant heat?

On earth, using earth-sheltering techniques can significantly reduce the temperature fluctuations on a structure. Would the same statement be true as well on the Moon? Does the Moon's core still ...
7
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2answers
1k views

If the moon was twice as big but twice as far away, would there be any difference?

I'm just going to go ahead and steal this question question directly of Reddit since I have more trust in the answers I get on this site. So, if the moon was twice as big but also twice as far way, ...
12
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5answers
4k views

Does Earth have a code name?

Everything we discover in the sky get eventually a code name, like NGC 7293, Simeis 147, etc. Does Earth/Moon have a code name too? Or it is just Earth/Moon, etc.?
6
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1answer
442 views

What is the percentage of stars with planetary systems

We have discovered quite a number of exoplanets to date. The Kepler spacecraft has examined 150,000 stars and found 1,059 exoplanets. We know that Kepler, as well as all other exoplanet searches to ...
2
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2answers
108 views

How to produce a loss-free combination of two “identical” beams?

This is for anyone with experience in optics/imaging/photography as well as anyone who likes to puzzle over tricky physics problems. As the title suggests, this is about combining two (for all ...
5
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1answer
3k views

How much sky do we see at any one moment?

When we look at any particular point the sky, what percentage of the celestial sphere do we see? This question arises from the notion that on average there passes one meteor per hour overhead. So ...
7
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1answer
67 views

Precision of spectroscopy for astronomy

How precise can the measurements be when looking at spectral lines in astrophysics? For example, suppose I have a telescope in orbit, and I am looking at $H_\alpha$ lines coming from a star at 613 ...
3
votes
1answer
64 views

Are there any “Neutrinian” standard candles?

Are there any theoretical models/investigations of object far away that could act as some type of neutrinian standard candles? That is, they spew out neutrinos of with some know characteristics that ...
3
votes
1answer
59 views

Are initial mass functions summable?

I tried to sum up two weight ranges of the IMF which wouldn't not work so my question is, if I'm doing something wrong. Let's say my weight ranges are $\left[X M_{\mbox{sun}}, Y ...
2
votes
1answer
263 views

Stable and Unstable Orbital Resonance

I was wondering if anyone can show me why some orbital resonances are unstable. For example in the asteroid belt there is a depleted distribution at 3:1 resonance with jupiter. What is the cause of ...
6
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2answers
664 views

Calculating the illumination of the moon

I am trying to find a formula that will enable me to calculate the illumination of the moon down to one thousandth of a percent, given that the Gregorian year, month, day, and hour is known. Can ...
8
votes
1answer
94 views

How close would Earth have to be for us to detect it was habitable, and then inhabited?

Given our current technology (or technology that is near implementation), how close would a clone of our Solar System (and so also Earth) have to be to us in order to detect that the cloned Earth was ...
2
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3answers
243 views

How early might a moon-sized asteroid be detected?

If an enormous asteroid, approximately the size of our moon (~2000-mile diameter), was passing close to Earth, how early might we detect something that large? Or alternatively, how close could ...
13
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3answers
452 views

Winter solstice, sunrise and sunset time

We all know the Winter Solstice comes on December the 20th or 21st, which is (by definition) the shortest day of the year. The Winter Solstice day is not the day of the year the Sun rises later (that ...
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3answers
2k views

Converting between Galactic and Ecliptic coordinates

I was hoping someone would be able to tell me the formula to convert between ecliptic and galactic coordinates. I've been able to convert values using ...