# Tag Info

1

There are photons traveling in all directions, not just the dozen or so you show. The further from the source the telescope is, the smaller the amount of solid angle it covers and the fewer photons it will gather. A $1 m^2$ telescope pointed at the sun will receive about $1.4 kW$. Taking a typical photon energy of $2 eV$ that is about $4.2E21$ ...

1

Assume a star the same mass as the sun. Now you have to figure out how far the sun has a gravitational influence, in theory this is infinite, in practice, lets take it as far out as the proposed Oort cloud, which is composed of icy planetesimals, perhaps like Pluto or large comets. I choose that distance because as far as I know, if this cloud exists, it ...

2

We know that 95% of the matter in the universe, and all around us, and in our room is dark matter. You are conflating dark matter and dark energy here. About 5% of the mass-energy of the universe is ordinary matter. Most (over 2/3) of that mass-energy is in the form of dark energy. The remaining 32% of this mass-energy is mass, or matter. About 84%, ...

3

The star acts as an "effective point source" since it is so far away and its angular extent is so small - in other words, the optical signal arriving at earth is "very nearly" a plane wave with the same phase over a large extent. This is what enables us to do interferometry. Think of waves, not photons.

1

You should not confuse the terms light years $[ly]$ to be a unit of time, it is actually a unit of distance, the distance light will travel in vacuum in one year. When we speak of the age of the universe we use years, in particular about $13.7$ billion years. Now do to the expansion of the universe the distance form which the light comes to our eyes or ...

0

Due to inflation distances does not correspond directly with time ago. 13.7 billion years ago is actually what you see at the distance of about 48 billion light years away. What causes this discrepancy is also the same thing that causes redshifts, so things that are more redshifted also have more of a discrepancy between the naive conversion 2bn ly = 2bn ...

3

In the "microscopic" sense, the formation of the Sun and solar system do not depend strongly on dark matter. Looking at things on the scale of GMCs, you can get a Jeans-unstable situation that will lead to star formation without invoking dark matter. There's a step much earlier in the history of the Universe that needs dark matter, though. In the early ...

5

Good answer from Kyle. I will just add that there is a great deal of effort going into trying to discover "solar twins". These are stars with such similar parameters (including age inferred from the HR diagram or asteroseismology, which can be good to about 10% in the best cases) and photospheric compositions to the Sun, that it is thought likely they must ...

16

You're right that the Sun being 4.5 billion years old makes observations difficult. The Sun goes around the Galaxy about once every 225 million years, so since the Sun formed it has gone around the Galaxy perhaps 20 times. The trouble is that the Galaxy is not like the Solar System: stars don't go around on nice nearly circular orbits, everything is a bit ...

8

To ask what do physicists expect to accomplish with gravitational lensing is nowadays somewhat like asking what do biologists expect to accomplish with looking at things with microscopes. Gravitational lensing is a well established method used across astronomy and the main challenges the field itself has to tackle are mainly technicalities. But I will try ...

1

NASA's JPL website has a fully referenced table that includes equatorial gravity (i.e. surface gravity at the equator). It looks like, for that table, they get the surface gravity by deriving it using a mass and radius rather than measuring it directly. However the citation for the mass of Venus is an article entitled Venus Gravity: 180th degree and order ...

4

This is not a complete answer to the question, rather a explanation of Kyle Oman's answer. When we (or at least me) think of superconductivity, we have in mind the pairing of electrons to form Cooper pairs. But this pairing is quite weak, and a moderate magentic field can destroy superconductivity. But electrons are not the only particles around! At the ...

5

Doubtful you'll find anything within the Solar System, but there are neutron stars, which are thought to have regions which are both superconducting and superfluid (that link is one of the original references from almost 50 years ago - there is a ton of literature on the topic since, you could start with some of these).

2

If you go to Wolfram Alpha and type in You get some very helpful information - the exact date and time of the next new moon (in the local time zone), as well as the moon rise and set times. Doing that right now, I get the following: You can decide if that is the information you need to compute the first day of Ramadan - but you can see here that if ...

2

There is one big advantage. The Earth would (virtually) never get in the way of the observing target, allowing for continuous long exposures on any target and greatly simplifying scheduling. The only other advantage I can think of is the possibility of getting parallax measurements slightly faster by using re-located-HST + a telescope on/around Earth ...

2

Have a look at this http://en.wikipedia.org/wiki/Astrophysical_jet I doubt if a jet can eventually settle down to a spiral galaxy. It needs to get rid of it's velocity, otherwise it will spread out rather than clump together; It needs to form atoms, which an electron ion jet cannot do; It needs to accumulate extra matter, a lot of it (and it's moving ...

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