22

Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain. The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is ...


19

Dark Matter Your understanding of dark matter isn't bad, but here's a few clarifying details. Orbits: The speed of an object's orbit is related to 2 things: the radius of its orbit and the mass inside of it. In the solar system, over 99% of the mass is concentrated at the centre, so radius is the dominant effect on orbital speed. As we look at planets ...


18

The main reason we don't see the bright center of our galaxy, which is composed of millions of stars, is dust. Visible light is absorbed and scattered by interstellar dust, but that doesn't mean we can't see it on other waves of the spectrum, for example, infrared light doesn't suffer as much because of the dust. Notice on this image how bright the galaxy ...


12

The answer here is certainly no. Many dwarf galaxies (e.g. the Magellanic Clouds) don't contain supermassive black holes, although some may contain less massive intermediate-mass black holes (Mezcua et al. 2018). As they are the most numerous type of galaxy in the universe, it's quite possible that most galaxies, in fact, do not contain supermassive black ...


10

One way to think of the Escape velocity is to imagine it backwards. Instead of a rocket being fired into space, think of the same rocket, starting at rest relative to the Earth at a great distance (for this thought experiment pretend the sun, doesn't exist and the rest of the universe is empty), then let it fall to the ground. No matter how far away you ...


9

The Sun orbits in the entire Galactic gravitational potential, which is very approximately similar to that provided by considering that all the mass interior to its Galactic orbit (about 100 billion solar masses) were concentrated at the Galactic centre. The black hole at the centre has a mass of 4 million Suns, so contributes less than 0.01% of the mass ...


8

Note: I am not an astrophysicist. If someone with the relevant expertise finds my analysis of the source paper to be invalid, I would appreciate your help with correcting my answer here. This is a somewhat complicated question to answer. Black holes may precede galaxies in the form of primordial black holes which according to some models arose in the early ...


8

A galaxy's center of gravity is not determined by the most massive object, but by all objects in the galaxy. Even supermassive black holes (SMBHs) do not dominate the gravitational field except very, very close to the center. By far, most of the stars in a galaxy couldn't care less about the SMBH. The region within which a BH dominates over that of the ...


7

The Sun's velocity is known (with uncertainties) with respect to the Galactic centre (GC), as is the Sun's distance from the Galactic centre. These measurements have a long tortuous history, which I am not going to attempt to summarise here. However, the motion of the Sun with respect to the GC is established by measuring the proper motion of the Sgr A* ...


7

The term you're looking for is called the bulge-to-disk (size) ratio. Sort of by definition, the answer depends on the morphology of the galaxy, i.e. how "late-type" spiral it is. "Sa" spirals are the ones that resemble ellipticals the most, and hence have large size ratios (of order, but below, unity), whereas bulges in Sc galaxies are (less than) one-tenth ...


6

This is a very well posed question, thanks! The problem is, we still don't know. What we suppose, is that dark matter came first. Primordial fluctuations of dark matter made the right conditions to accrete enough mass to build SMBHs, and then the surrounding galaxy. But still we don't know if, the SMBH comes from merging, of other SMBHs or galaxies, or ...


6

Most of the light energy in the universe is still in the cosmic microwave background (CMB). Spring 2011 UC Berkeley Physics 250 class materials calculate from the fact that $T=2.73$ for the CMB: It follows that photons contribute only $0.0000485$ of the closure density. Closure density is the density needed to close the universe if there were no ...


6

No, it seems that the formation of a supermassive black hole is a consequence of galaxy formation, not a cause of it. The mass of the black hole is a very small proportion of the mass of the matter and dark matter in the galaxy. The details are not yet worked out but it is the distribution of dark matter that forms the galaxy, and then galaxy mergers ...


6

James K's answer is great, I just want to offer a few definitions: Any mass $M$ — whether a be point mass like a planet or an extended mass like a galaxy — has an associated gravitational potential $\Phi(\mathbf{x})$. This is defined as the energy needed to bring a unit mass from the point $\mathbf{x} = \{x,y,z\}$ to infinitely far away from $M$. The ...


6

It's generally assumed that most galaxies have a black hole at their centre. There are galaxies we've detected that don't seem to have a black hole though, so it's not a necessary requirement or a guaranteed find in every case. In cases without a black hole, it has been postulated that the gravity of the stars and gases combined is enough to hold the galaxy ...


6

To answer the title question, what you want to know is the bulge-to-total ($B/T$) ratio, which is the fraction of a spiral galaxy's light (and thus, approximately, its stars) which in the bulge; this ranges from 1 (it's all bulge, nothing else there -- i.e., it's an elliptical galaxy) -- to 0 (no bulge at all). Consistent with pela's answer about sizes, the ...


5

As barrycarter already pointed out, it is due to the huge time it takes for the milky way to rotate. For once, as he pointed out, the stars in an orbit similar like the sun take about 250 million years for one full revolution around the center of the galaxy. If you put that in a relation to an earth year and take our civilization of roughy 6000 years of ...


5

Star S2 reaches maximum velocities of 5,000 km/s according to this ESO page. The orbital period is given as a little over 15 years and this paper gives a peri-center (closest approach to the black hole) of 17 light hours. There's another star in the area called S0-102 which has a shorter period, but its orbit is less eccentric - so its closest approach to ...


5

The stars in the Galactic bulge are predominantly metal-rich (by that I mean have a metallicity similar to the Sun or even a little higher). Even though these stars are predominantly old, the bulge is thought to have formed extremely quickly and the interstellar medium from which the stars were formed would have been enriched with metals very quickly. Here ...


4

The Sun orbits in the Galactic potential. The motion is complex; it takes about 230 million years to make a circuit with an orbital speed of around 220 km/s, but at the same time it oscillates up and down with respect to the Galactic plane every $\sim 70$ million years and also wobbles in and out every $\sim 150$ million years (this is called epicyclic ...


4

All quoted text in this answer is from image captions in the Wikipedia article on the Milky Way. 360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO This magnificent 360-degree panoramic image, covering the entire southern and northern celestial sphere, reveals the cosmic landscape that surrounds our tiny blue ...


4

You are mostly empty space. Every atom in your body is very tiny compared to the relatively vast spaces between it and its nearest neighbors. And the same goes for every "solid" object you ever saw. But when you look at yourself in a mirror, the number of atoms you see is so incredibly vast that you don't see the spaces between them but instead ...


4

Milky way obviously is an English word with that literal meaning a way (path) covered in milk. So it's not exactly derived. But it is a literal translation from Latin 'via lactia' and the Greek γαλακτικός κύκλος (milky circle - galaktikos kýklos, hence the word Galaxy). It resembles our own Galaxy's appearance on the night sky as a diffuse, slightly milky ...


3

It has to do with the formation of the Milky Way. At the beginning, the Milky Way was much more spherical than it is now - perhaps closer to what an elliptical galaxy is like than a spiral galaxy. Population III stars would have formed first, then quickly died out. Next came Population II stars. They formed when the galaxy was still somewhat spherical, and ...


3

As Yashbhatt said, we can detect light: with our eyes (visible light only) and with special machines. We can also see the effects of some type of lights. Dark matter, however, cannot be detected for now. Also, light is energy, dark matter is matter. Why does your skin tan? It's because of the ultraviolet light. Why are you hot each summer? It's because of ...


3

Light may account for a small portion of dark matter, but it is unlikely to account for most/all of it. From a Wikipedia article on dark matter: http://en.wikipedia.org/wiki/Dark_matter the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3] Thus, dark matter is estimated to constitute 84....


3

As @Py-ser already said, there is the very clear information about that, although his link correctly is here. As we can read there, the distance of this is around 25000 light years, with a precision of 1400 light years. Normally, the size of most galaxies is around some ten thousands of light years. Compared to the black holes, their size (the diameter of ...


3

The galactic bulge is formed by tightly packed stars and interstellar dust Also most stars are in the direction towards the galactic center Thus it would appear brightest if it were viewed from outside the plane of the galaxy as this image is intended to show. However, we don't see this from Earth, and the reason why is explained in Phys.org's Why can't we ...


3

What they did was the following: for each individual filter, they assembled overlapping frames into a combined, single-filter image. E.g., they combined several g-band images into a single g-band image, combined several r-band images into a single r-band image, etc. (So they ran SWarp five times, once for each filter.) They then put each combined single-...


2

The ones in the halo have blue colors, which is the "old" part of the spectrum. Each type of metallicity is usually measured using a form of logarithmic scale, since the flat percentage of metals (especially iron peak elements) of any star will be very small. In this case, we have $$[Fe/H] = \log(N_{\text{Fe}}/N_H)_{\text{star}} - \log(N_{\text{Fe}}/N_H)_{...


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