# Tag Info

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The highest resolution 3d printers I know of are around 1600dpi, which is a resolution of about 15$\mu m$. Telescope mirrors have to be smooth to fractions of a wavelength of light, so the resolution of current printers is nowhere near good enough. Whether 3D printers could one day be good enough is a different question, but given that the improvement in ...

21

A typical giant galaxy, such as the one you've provided a picture of, has a radius of something like $10\;\rm kpc$ (kiloparsec - $1\;\rm pc \approx 3.2\;ly$). A supermassive black hole hosted in such a galaxy has a mass of something like $10^6-10^9\;\rm M_\odot$ (solar mass, $1\;\rm M_\odot \approx 2\times10^{30}\; kg$). The monstrous billion solar mass ...

4

Yes, that can happen. It is somewhat realized in positronium, a bound electronic state where an electron and a positron revolve around each other. Both have the same mass, so they could (classically) have the same spherical orbit. With Newton, you have an attractive force for two equal bodies of mass $m$ of $$F = G \frac{m^2}{d^2}.$$ The centripetal ...

4

You are neglecting two important facts. The first one is that stars, toward the end of their lives, return to the interstellar medium (ISM) a lot of their initial mass, but now enriched with heavy elements produced by nuclear reactions inside the stars themselves. In this way, younger stars which form from the ISM begin their life with a larger fraction ...

4

A brief overview of stellar evolution can be depicted in the following image: (From here which says it is originally from an encyclopedia; click here for larger image). The heavier stars (top track) have very short life times (a few million years) because they run through hydrogen, helium, carbon+oxygen, ..., iron fusion in the core. Once a particular ...

3

I will select quotes from the wiki article on structure formation, bold mine: The very early Universe In this stage, some mechanism, such as cosmic inflation, is responsible for establishing the initial conditions of the Universe: homogeneity, isotropy and flatness.3[6] Cosmic inflation also would have amplified minute quantum fluctuations ...

2

Have a look at the Wikipedia article on Tests of General Relativity. The most recent measurement quoted there was by a group from the University of Texas. You can find a copy of the paper here. They measured a deflection of $1.66$ arcseconds $\pm 10\%$, compared to the prediction from General Relativity of $1.75$ arcseconds. Respond to comment: The ...

2

Can't be done, yet. But don't expect 3d printing to stand still, it's only recently been born, wait till it starts walking, and running! My hope was that there could be a vacuum process that was usable in just enough vacuum to form a perfect mirror shape over a perfect mirror mold of some kind, then start the vapor deposition on the cheap, lightweight ...

1

Gyrochronology is semi-empirical in the sense that there is some justification for the temporal dependence that you mention. There is a line of argument for the $t^{1/2}$ dependence and it can be found on pp7-8 of this pedagogical review by Jerome Bouvier. http://arxiv.org/pdf/1307.2891v1.pdf The basic idea is of a spherically symmetric, ionised wind that ...

1

It is difficult to keep track on these things, specially since quasars are also highly variable. Trying to answer the question of the post title, I found this example that sound pretty impressive: $7\times 10^{14}\,{\rm L_\odot}$, or $1.4\times 10^{41}$ W. The ESO press release refers to the kinetic luminosity, or the kinetic energy of the outflow per unit ...

1

Two equal mass bodies orbiting around their centre of mass is exactly how Newtonian gravity works. Your analogy of the runners is actually pretty good. Think about it this way - the two bodies are always attracted toward each other, so they accelerate a bit in that direction. Notice that "toward the other body" necessarily means "toward the centre of mass". ...

1

As of July 27, 2014, Google still says the Sun is at RA 19h 4 m 31 s, Dec 63° 52.200': That's so embarrassing, Google! So what are those coordinates? They obviously aren't the coordinates of the Sun from a geocentric perspective. The geocentric right ascension and declination of the Sun vary with time. Those coordinates are instead the coordinates of the ...

1

A recent paper suggests a mechanism to create totally metal stars. Some Stars are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies http://arxiv.org/abs/1406.5509 Ordinary star formation produces planetary mass objects (sub-brown dwarfs), so perhaps this new mechanism would also do so. ...

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