Tag Info

New answers tagged

4

Both elliptical and spiral galaxies have a stellar mass profile which have a well-defined maximum. In addition, both types of galaxy have symmetries - axial in the case of spirals and either axial or triaxial in the case of ellipticals. Since, in general terms, the luminosity tends to follow the stellar mass, this means that their projected brightness on the ...


1

If I understand your problem correctly, the diagram looks like this: Where I am drawing on the unit sphere, so distances $a$ and $b$ scale from 0 to $2\pi$. The sine rule for spherical triangles is $$\frac{\sin{A}}{\sin{a}}=\frac{\sin{B}}{\sin{b}}$$ Now since $B=90°$, it follows that $$a = \sin^{-1}(\sin{A} \sin{b})$$ Please see whether that makes ...


3

I think the following image, which comes from Tomczak et al. (2014) and the so-called ZFOURGE/CANDELS galaxy survey should do the trick. It shows how the galaxy stellar mass function (i.e. the number of galaxies per unit mass per cubic megaparsec that have a certain stellar mass) evolves as a function of redshift. As you might imagine this is not just a ...


3

The angular resolution of a telescope is given approximately by $1.22 \lambda/D$ in radians, where $\lambda$ is the wavelength of observation and $D$ is the diameter of a circular mirror. Say we study a star that is 10pc away with a telescope working in the optical band (you didn't specify) at 500nm, then the spatial resolution of a 100m telescope at the ...


2

A recent paper by Dan Whalen titled, Finding the First Cosmic Explosions. I. Pair-instability Supernovae discusses this very problem. The pair-instability supernova (PISNe) is a special case of massive stars, around 100 $M_\odot$, in which the thermal pressure inside the star is reduced via the production of electron-positron pairs. Runaway thermonuclear ...


1

Firstly, it is important to note that the old Big Bang cosmology is no longer the most widely accepted theory. We include inflation into the mix in current theories. That said, there is an ambiguity in the definition of the Big Bang (you can find information on that in my question here). If we take the definition of the Big Bang as coming before inflation, ...


1

Its complicated. There wasn't just one Hubble Ultra Deep Field (UDF) observation, but several, taken at different times, with different instruments, but pointed in (almost) the same direction. The first image you refer to was taken with the WFC Infra red camera in 2009, in near infrared bands (1-1.6 microns). The area covered by this camera is $2.4$ ...


4

Your method is correct. When the angle is "small" so that we can ignore curvature, then the rectangular solid angle is just the product of the two side angles. (This doesn't hold for "large" angles). I cannot find a perfect explanation, but one source of this confusion may be that the UDF was imaged by two separate instruments, one optical, one ...


0

The expansion of space due to "inflation" is considered to be the cause of significant redshifts that is observed. It is sometimes referred to as a "Cosmological redshift", and is described with the appropriate equations.


5

In this link, the contradiction is "explained": The tremendous expansion greatly dilutes any initial curvature. Think, for example, of standing on a basketball. It would be obvious that you are standing on a (2-dimensional) curved surface. Now imagine expanding the basketball to the size of the Earth. As you stand on it now, it will appear to be flat ...


0

To date there is nothing published (and serious) that makes a confirmed detection of dark matter particles. Thus, the only evidence in favor of its existence remains from indirect methods: calculate the mass that should be there based on visible sources (stars, galactic powder, etc), and use this mass to calculate the speed of stars about the galaxy as the ...


0

A good, semi-technical discussion of the general problem (how the post-Big-Bang evolution of the universe, including the formation of galaxies, stars, etc., can be reconciled with the 2nd Law) can be found here: http://arxiv.org/abs/0907.0659 It's important to realize that while the ensemble of atoms in the gas cloud does indeed, as your intuition suggest, ...


0

Astronomy is mostly concerned with observing the night sky, calculating the positions and movements of the heavenly bodies and identifying objects. Astrophysics is concerned with figuring out how stars form, studying the chemical reactions within the stars, calculating what elements the stars contain and so on.


5

I've added this because I don't think the accepted answer is very clear. Estimating the number of stars in the Galaxy relies mostly on two things. We estimate the present day mass function (that is the number of stars that exist per unit mass per unit volume) in the solar neighbourhood. We construct a model for the overall density distribution of the ...


0

This question confuses two frameworks, the classical thermodynamic one with the quantum mechanical one. A perfectly homogenous radiating body belongs to classical thermodynamics. Photons belong to quantum mechanics, as well as the uncertainty principle. Lets look at the problem classically. Electromagnetic radiation is continuous from a classical body ...


3

Assuming you are talking about exoplanets, I'll offer this. To obtain a density you need a mass and radius. Masses come via two methods - either measuring the radial velocity variations of the star it orbits (the bigger the RV variations, the bigger the planet mass), or so-called transit timing variations. This latter works in multiple "transiting planet" ...


3

There are several concepts and issues going on here that are getting mixed together. First the ADC; An ADC (Analog To Digital Convertor) takes signal values in a certain range and converts then to a digital number. Let's run some numbers: A N bit ADC will be able to represent 2^N states. So a 4 bit ADC can represent 16 distinct states, 8 bits => 256 ...


3

Very simply, some fraction (usually 50-80%) of photons impacting each pixel on the CCD will result in an electron being excited out of the photoactive silicon layer and adding to the charge on a capacitor associated with that pixel. As the number of photons increases, so does the charge on each capacitor, in an approximately linear way. When the CCD is ...


-1

ADCs - analog to digital converters. Consists usually from several "more-or-less" units, called "comparators". Usually comparators compare electrical current. No, this current is not accurately correspond to number of photons which drop on ccd device. CCD matrix is complex system which works as "multiplyer" device. Many p-n layers are used which are ...



Top 50 recent answers are included