Starship Asterisk*

Hello, why does Earth show diffraction spikes but the Moon does not? Is it because Earth is brighter than the Moon and Earth is overexposed? Also, other images of Earth from Cassini show Earth as faint blue dot. But here Earth and Moon both appear to be of the same color. Is this because this is a r...

I sort of understand how a moving observer could not see itself crossing an event horizon. But the stationary observer dropping the object in? What's that about? An outside observer does see objects cross the event horizon. If you yourself fell into a black hole, however, it would take infinitely l...

I think ( a hyphothesis), a black hole is not a "hole". I think that in the center of a black hole there is a " black star" of neutrons. This "black star" of neutrons has more density, diameter, magnetic field and gravity than a pulsar. It rotates at high speed as a pu...

It makes me wonder if the other force carriers are similarly altered in that area where gravity is so dominant. That’s probably know; but not by me. The other fundamental forces, the strong and weak nuclear force, are altered mathematically in the same way as electromagnetism around a black hole. T...

In the analogy of a waterfall, wouldn't the ground the water flows over represent the spacetime that is not moving? The ground is not the reason why the swimmer is pulled over the edge, but the flowing water. The flowing water represents the gravitational pull caused by the curved spacetime. It was...

Just what the heck is "space", really?? This discussion goes back to the days of Newton an Leipniz. Newton argued for an absolute space, while Leipniz insisted the only thing you can measure are the positions of objects relative to each other. My quick answer is that you cannot measure sp...

Nice work Rhys and Rory!

Is there a reason why the central bulge starts to move to the "upper right"? Naively, I would imagine that the center of gravity stays at the center.

But if everything is flying apart how can they collide? Not everything is "flying apart". If you mean the expansion of the universe, it happens on a much larger scale (on the scale of superclusters of galaxies). On the scale of a few million light years (which is small compared to the siz...

Can someone tell me the size of the Horsehead Nebula?

I thought the redshift was measure because we know from how much the carbon spectral line has been 'reddened'. But how comes we can figure it out in a simple gamma-ray burst? I thought they cannot show any spectral line, or do they? From what I understand from this paper , they use the absorption o...

If the table head included units, it would be perfect. Thank you APOD of sharing this!

It is my feeble understanding that inside the event horizon, space-time gets so wacky that time replaces the space dimension and forward time points toward the center. Stuff can't get out, except Hawking Radiation, because for the same reason we can't go back in time. This is indeed another way to ...

A fascinating thought is whether or not spacetime itself is capable of "rupturing" right next to a black hole that is spinning perhaps as fast as the speed of light. Ann Spacetime doesn't "rupture" at the event horizon of a black hole. At the "center" of a black hole, ...

Wouldn't it be more efficient to land on the asteroid and use the thrust to push directly on the asteroid?? According to the authors, a propulsion system would have to be anchored to the asteroids. But this is difficult because asteroids are "rough and unconsolidated". Another problem is ...

According to the paper, it would take about 20 years to deflect a 200m asteroid. Since the space craft needs to get there too, which would probably also take a few years, we really need "sufficient warning and time".

The link to the reflection nebulae is missing a ".html". Otherwise, really nice picture!

The most noticeable such constant: the fine structure constant ~ 1/137 => 0.000 The world gets along just fine with a small fine structure constant; having a zero small fine structure constant would make little difference. Having a vanishing fine structure constant means that there would be no elec...

Definitions of space-time are highly arbitrary and are mostly for calculations, bookkeeping & conceptualization. What is NOT arbitrary (i.e., what is actually physically real ) are the Ricci and Riemann curvature components of space-time. What is arbitrary and relative are the coordinates used....

Space IS disappearing into black holes; I disagree. The spacetime of a Schwarzschild black hole, as described by the Kruskal-Szekeres coordinates, is static. What does change with time is the position of free falling particles and photons, which move on geodesics. The waterfall analogy mentioned in...

I never understood why the ending of calenders in December implies the end of the world. Up to now, my calenders always ended in December and I could always buy a new one that starts in January.

The Earth, with its shorter orbit, approaches Jupiter each year, NOT the other way around!!! Try for some ACCURACY, PLEASE. Dear 5djw28, since English is not my native language, I looked up the meaning of approach at Merriam-Webster . I find the following two examples helpful: - The cat approached ...

It is one thing to see a picture of a shuttle at the launch bay. But seeing it next to a real house gives me an impression of the sheer size of it.

[...] return signal of 0.003 photons for a single 1e32 photon isotropic pulse. The site doesn't like that answer, so either the site is wrong, or I'm misunderstanding the question I made the same mistake at first. They require you to round to the nearest whole number , which is 0 in this case. Stil...

Distance to HTcas: R = 140*3.09*10^16 m = 4.33*10^18 m Number of emitted photons: N = 10^32 Number of photons per square meter = N /(4*pi*R^2) = 4.25*10^-7 Arecibo's collecting area: A = 7854 m^2 Number of photons on Arecibo's collecting area: n = A*N /(4*pi*R^2) = 0.003 Rounding to nearest whole nu...

Moonlady wrote:

orin stepanek wrote:New Horizons is 24 AU distance from the sun; less than 8 1/2 AU to go.

What does AU mean? Thanks

AU stands for Astronomical Unit, and is the mean distance between the Sun and Earth.