All Questions

33 views

Could a cryo-volcano be the reason behind this colour difference in Iapetus's hemispheres?

Iapetus's hemisphere facing Saturn is dark, whereas the opposite one is bright. Could a cryo-volcano be the reason behind this colour difference in Iapetus's hemispheres?
94 views

Is lunar elevation at a given location for a given day unimodal?

Is lunar elevation at a given location for a given day unimodal: Unimodal function In other words, once the moon's elevation reaches a minimum (which may be above or below the horizon depending on ...
109 views

USNO moon images look quite different?

I visited: http://aa.usno.navy.mil/imagery/moon http://www.usno.navy.mil/USNO/time/moon-phase-images within seconds of each other, and saw two very different "current moon images": Is one of ...
204 views

Does the radio signal decay when it travels through the intergalactic space?

When you emit the radio signal it starts moving at the speed of light. Radio beam is diffusing with each kilometer the signal has traveled. To the nearby receiver the signal is strong. But if the ...
71 views

Does Ceres have an exosphere similar to the Moon?

Thinking about the question and answers for Does the Moon have any oxygen in its atmosphere? begs the question, despite being considerably smaller than the Moon and further away from the sun, does ...
158 views

What is the object in this photo?

What object is in this photo? View coordinates: $47.25103 \ \ 38.81697$ Time: $2013$-$11$-$23 ~21$:$00 +4 ~UTC$ Sony $A580, \ 50mm f/1.4 15"$
110 views

Apparent size of M31

I am trying to understand something I read on wikipedia about M31. Wikipedia says that M31 appears more than six times as wide as the full Moon. But I remember that when I watched it naked eye it ...
77 views

Appearance (popularity) ranking of stars?

Is there some ranking / statistics that shows how common given type of celestial objects is? A table that would allow the answers to questions like: What type of star is most common per volume of our ...
910 views

How many stars and galaxies can be seen naked eye?

How many of the luminous dots that we see naked are galaxies and not stars from our galaxy? I immagine that the majority of the luminous points that we see naked eye during the night, are actually ...
485 views

How does one determine the effective temperature of a star from its spectrum?

Determining effective temperature of a star is in general a non-trivial task. Simple reason for this is that we can only study the electromagnetic radiation from a star, but not the temperature ...
171 views

Why is the observable Universe larger than its age would suggest?

The age of the Universe is estimated at 13.8 billion years, and current theory states nothing can exceed the speed of light, which can lead to the incorrect conclusion that the universe can't have a ...
113 views

Can moon set after being up when due north? (pseudo circumpolarity)

I'm in the northern hemisphere, facing north, and notice that the moon is due north (azimuth 0) and is still above the horizon, albeit barely. Does this mean the moon is necessarily circumpolar ...
96 views

How well can we in principle determine $T_{\textrm{eff}}$ of a star?

This is a question about the basics of astronomy, which I have never happened to see a good discussion for. It is about how well would we be able to measure effective temperature of a star, if we had ...
422 views

Is C/2012 S1 (aka ISON) the fastest comet on record to-date?

This source may be nebulous (no pun intended!) for some. It writes to say Starting at around 95,000mph at the start of this month, Comet ISON will eventually reach an astonishing 845,000mph on ...
551 views

How do scientists determine the age of stars?

I was Googling about the oldest galaxies in the Universe. Everywhere is written that their age is known by the light. Referring to this line "Since light travels at a set speed, if you look at a star ...
304 views

Does a planet's mass or gravity affect the height of it's mountains?

According to this Wikipedia page, the top five highest mountains on Mars (and the highest on Venus) are all taller than Mount Everest (and Mauna Kea as measured from the ocean floor). Does a planet's ...
147 views

Does our universe have to be embedded in a higher dimensional space?

I've heard that our universe may be open or closed. If it's closed it might have a toroid shape. If this is the case, would that imply that our 3 spatial dimensions have to be embedded in a higher ...
3k views

What's the fastest moving object in the universe?

We know that nothing can have proper velocities larger than the speed of light in vacuum. But are there any objects in space that get close to it? Any comets, or other objects thrown by gravity or ...
112 views

What do we know about the lifecycle of the Milky Way (or any other spiral galaxy)?

I know that the Milky Way will collide with Andromeda in the distant future but based on what we know so far there is a supermassive black hole in the center of each galaxy and thus the Milky Way will ...
96 views

How can only 1 magnetic pole of the Sun change at a time?

I read some news stories about the current Solar maximum and the flipping of the magnetic poles. They say that one of the magnetic poles has switched polarity and the other will switch in about a ...
175 views

Can I see comet ISON from Saudi Arabia?

I live in western Saudi Arabia, I am trying to see the comet ISON, reading here and there tells that I can see it by naked eye somewhere next to the constellation leo, I used an app to tells me where ...
214 views

Inflation cosmology: slow-roll inflation versus tunneling between two vacua

In slow-roll inflation models, the early inflation of the universe is driven by the flat non-zero part of the inflaton potential, and it ends as the ball rolls down the cliff and the potential energy ...
89 views

How can the equation of state for cosmic strings and domain walls be derived?

In this article which nicely explains why it is really the quantity $\rho + 3p$ which is relevant to determine if the expansion of the universe is accelerating or decelerating by making use of the for ...
48 views

Why is there billowing dust in nebulae?

Why do some nebulae look like the billowing clouds? Wouldn't that require something like a viscosity in empty space?
161 views

Calculate Distance To Stars

I was just watching a lecture from Carl Sagan. He talked about figuring out the distance to the stars; it got me interested in learning more about the subject. As far as I know, the Inverse square ...
126 views

Will Neptune be visible with the naked eye if I am standing on its satellite

Assume that I am standing on one of the many satellites of Neptune. Will I be able to see the planet with my naked eyes (without any sort of visual aid). If I was to stand on Moon I would be able to ...
148 views

How does a Bahtinov mask work?

For focusing the image of a telescope, one can use a Bahtinov mask. How does this mask work, and how did Mr Bahtinov get the idea to cut such a peculiar shape? Is it possible for me to design my own ...
54 views

Crab Nebula Time Period

I've learned that the time period for a rotating neutron star, i.e., a pulsar is very accurate and can be used as an interstellar clock. But according to Walter Lewin's 8.01 Lec 19 on classical ...
65 views

How are the newly discovered Janus/Epimetheus rings different from the other rings of Saturn?

What are the newly discovered ring systems of Saturn, and the circumstances relating to the discovery? Is there something that makes them different from the old well-known rings, like their formation? ...
51 views

Open data for satellites positions

Is there a site / service where I can get the data for the positions of satellites (I mean artificial satellite, ISS, etc.)? I know that some sites provide a visualization of these positions, but I'm ...
46 views

Solar maximum and minimum

What are solar maximum and solar minimum? Is there a relation between sun spots and solar maximum and minimum? Thank you!
78 views

How can we be sure that we have identified very distant stars correctly?

From reading this question on calculating distance to stars and from a bit of background reading on the standard candle theory I still don't see how we can confirm that a star we see at one distance ...
323 views

What does it mean for a star to go nova or supernova? Can I safely observe these?

What does it mean for a star when people say it goes 'nova' or super nova, what are the differences? More importantly, can I safely observe these with an amateur telescope? I imagine they would be ...
150 views

Why do black holes have jets and accretion disks?

If supermassive black holes possess the gravity required to prevent the escape of photons and other mass particles, why do jets and accretion disks form? They seem to indicate that something is ...
255 views

How can I calculate moon phase and height?

How could I know what phase the moon is in when it's high in the sky during sunrise? How could I tell in general what its height in the sky will be at any given time?
47 views

Do we live in a galactic bubble?

I have heard a certain statement, which can be summarized succinctly into "we live in a local bubble", implying that solar system is located in an intestrellar dust underdensity region. This region ...
47 views

How can I measure the mass of eclipsing binary stars?

Is there any method to measure the mass of eclipsing binaries without using Doppler shift effect?
168 views

Colossus telescope, trying to outsmart aliens?

I was listening to Jeff Kuhn's talk on SETI's Colossus telescope project. Background: He explains his theory that a civilization living somewhere the galaxy would want to hide, and thus would ...
102 views

Planned telescope to detect alien waste-heat

I remember watching a talk by someone who designed a telescope to view a single star at a time. The purpose of the telescope was to detect waste-heat which would presumably be emitted by any ...
117 views

What Causes the Large Radiation Fields Around Jupiter?

I had heard that the large radiation belts around Jupiter may be formed by liquid metallic hydrogen in (or around) Jupiter's core (which Wikipedia says haven't been observed in labs yet due to the ...
2k views

How many sun-like stars are there in the universe?

After yesterday's announcement of the Kepler telescope finding a huge amount of newly observed exoplanets, i saw a headline claiming that as much as 22% of sun like stars in the universe have planets ...
223 views

Is there a upper limit to the number of planets orbiting a star?

Our sun has 8 planets orbiting as well as a number of dwarf planets. Are there any calculations that hint as to whether this number is close to some theoretical maximum value or are we simply an ...
142 views

What happens to a star after it has become a red giant?

When our sun reaches the end of its lifetime , it will turn into a red giant. How long will it be a red giant, and what happens after a star has been a red giant? Does it turn into a planet after a ...
113 views

What are the next planned space telescopes?

We've had hubble for two decades. Its become the most famous space telescope, and lately Kepler seems to be running for the second place. Are there any more powerful, better telescopes planned for ...
433 views

Most important feature of a telescope

I was taking an astronomy test, the following question came up: The most important function of the astronomical telescope is: Resolving power Light gathering power magnifying power ...
164 views

How can I safely observe a Solar Flare?

Solar Flares obviously release extreme amounts of energy and extend thousands of miles out into space. Because they are so big I would like to be able to observe some of these events through a ...
113 views

Is there a natural process by which hydrogen is generated from heavier elements in the cosmos?

we know that stars fuse hydrogen into helium starting at 3 MK; 13 MK in the Sun's core; carbon fusion starts at above 500 million K, and silicon fusion starts at over 2700 million K for comparison; ...
63 views

What is the direction of a comet's dust tail before and after perihelion?

I recently created a visualisation of comet ISON's orbit. However, I received a critique that the dust tail of the comet should be pointing away from the Sun after perihelion. As far as I know, the ...