83 votes
Accepted

Is there a better explanation of this picture showing the very distant star "Earendel"?

I did the annotation of that figure for the press release, so let me start by apologizing for the poor explanation, and then try to dig deeper into what's going on :) (although you already seem to ...
pela's user avatar
  • 38.2k
32 votes
Accepted

If the Sun was replaced with a sun-mass black hole, would it be visually detectable?

Yes, easily with a telescope, but not with the naked eye. It is a matter of routine to detect the 1.7 arcsecond shifts caused to stellar positions when seen near to the limb of the Sun. The removal of ...
ProfRob's user avatar
  • 152k
29 votes
Accepted

Is it possible to detect gravitational lensing of stars behind the Moon?

Measuring the gravitational deflection of light by the Moon is just out of reach of current observational techniques. The angular deflection caused by the lensing of a distant background object by a ...
ProfRob's user avatar
  • 152k
26 votes
Accepted

If dark matter bends light, how do we know the stuff in the sky is where we think it is?

The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ ...
HDE 226868's user avatar
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25 votes
Accepted

Was the Sun's gravitational lensing observed in other solar eclipses than the one in 1919?

Yes, observations of this kind are within the technical scope of amateur astronomers. Several groups succeeded in replicating the experiment during the 2017 eclipse that crossed the USA. For example ...
James K's user avatar
  • 121k
22 votes

Does seeing a gravitationally lensed/magnified galaxy imply that they could also see us as well?

It is highly unlikely that a galaxy that we see as magnified by lensing is, has, or will be positioned to see our galaxy similarly magnified. The three galaxies, including the lensing galaxy or ...
eshaya's user avatar
  • 3,726
19 votes
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If you were on a planet orbiting a star that has a black hole companion, could you see light from your host star bent 180 degrees?

The answer is yes. Roughly speaking, for a spinless Schwarzschild black hole, light that gets to a closest approach to the black hole of just a little greater than $1.5 r_s$, where $r_s$ is the ...
ProfRob's user avatar
  • 152k
16 votes

How can gravitational lensing makes a quasar appear brighter?

The quasar gives out light in all directions. The light spreads out in space. Only a very small amount of that light would be pointed exactly in the direction of your telescope. But if a large galaxy ...
James K's user avatar
  • 121k
16 votes

How do we know that objects that appear in duplicate or triplicate, etc. due to strong gravitational lensing aren't actually multiple objects?

Perhaps this isn't the case for every scenario, but I can think of at least two instances where this can be determined: Stars In the case of stars, it's pretty straightforward to get the spectra of ...
Justin T's user avatar
  • 3,404
15 votes
Accepted

Are astronomers waiting to see something in an image from a gravitational lens that they've already seen in an adjacent image?

What you do is cross-correlate the observational datasets for the multiple sources and look for the "lag" that maximises the cross-correlation function. Generally speaking, the "events" are not really ...
ProfRob's user avatar
  • 152k
14 votes

Would be possible to detect Planet Nine as a microlensing event along the line of its projected orbit?

The planet would not be able to micro-lense light from distant stars, but it could in principle obscure a star along its path. In order for gravitational lensing to work, the light path from the ...
planetmaker's user avatar
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11 votes
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Is it possible to observe strong gravitational lensing with amateur telescopes?

The cosmic horseshoe is beyond amateur instruments. It is a magnitude 20 object. In a large (2.5 m) professional telescope it looks like: This image taken from the SDSS III data. It is small (10'', ...
James K's user avatar
  • 121k
10 votes
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What is the gravitational lensing focal distance of a white dwarf star?

The gravitational focus you are talking about is actually a minimum value, defined by parallel rays of light from a very distant star just skimming past the Sun as they are bent according to General ...
ProfRob's user avatar
  • 152k
9 votes

Images of gravitational lensing

As mentioned in the comments, a ring will (ideally) only occur if you have perfect alignment of source, lens, and observer. This is true for point sources, for extended sources things get more ...
Alex's user avatar
  • 2,265
9 votes
Accepted

Has GAIA learned anything about General Relativity looking near Jupiter? (Gerry Gilmore: "oblate rotating mass moving in a deeper (Solar) potential")

Gaia is the follow-up to the Hipparcos mission, which was deactivated in 1993. So Gaia's results are decades in the making. The word "Gaia" was originally an acronym for "Global ...
Daddy Kropotkin's user avatar
9 votes

Gravitational mirroring: Can we theoretically see the Milky Way using a telescope?

In theory, it's possible, but in practice it's extremely unlikely. The deflection angle is small unless the light ray passes very close to the centre of mass of a compact lensing body. From Wikipedia, ...
PM 2Ring's user avatar
  • 14.4k
9 votes

Does seeing a gravitationally lensed/magnified galaxy imply that they could also see us as well?

It depends on the distance between us, the lens galaxy, the magnified galaxy, and many other factors due to the way light travels. A lens galaxy passes the old light of a far-off galaxy and amplifies ...
4NT4R3S's user avatar
  • 899
8 votes
Accepted

If I can't unscramble an egg, how do Astronomers unscramble views gravitationally lensed by complex mass distributions?

I work in gravitational lensing, so maybe I can give you an idea. In those JWST you have a massive galaxy cluster that is bending the light behind and acting as a true lens. If you have a temptative ...
JOAQUÍN HERNÁNDEZ's user avatar
8 votes

Is gravitational lensing, optical in nature or topological?

I think it is better to say that gravitational lensing is geometrical in nature. Topology deals with how spaces are connected. It entirely ignores the metric, how distances are defined in the space. ...
Anders Sandberg's user avatar
8 votes
Accepted

Would be possible to detect Planet Nine as a microlensing event along the line of its projected orbit?

With the caveat that it is easy to make an arithmetic blunder, I calculate that there is around a 1 in 25 chance per 6-month interval that planet 9 might cause a significant microlensing amplification ...
ProfRob's user avatar
  • 152k
7 votes
Accepted

Would gravitational waves be subject to external gravitational perturbations?

Firstly, gravitational waves (GWs) are not an echo - we measure the direct signal. The process you describe here is known as gravitational lensing, the deviation of (usually) light rays due to ...
AstroFloyd's user avatar
  • 1,044
7 votes
Accepted

How are gravitational lensing and dark matter related?

Gravitational lensing of background galaxies and quasars is used to probe the amount of gravitating matter in foreground objects such as clusters of galaxies. Here is an example, taken with the ...
ProfRob's user avatar
  • 152k
7 votes

Is it possible to see stars that are gravitationally lensed by the sun from the ground?

The answer is no. But there's an interestign alternative. The Eddington's experiment was not about seen a gravitationally lensed star but to see a small displacement of its position (just below 2 ...
Swike's user avatar
  • 3,916
7 votes

Is it plausible to use other stars for the proposed FOCAL mission instead of the Sun?

Gravitational lensing works from anywhere beyond the focus, so in that sense, we could use any star as a gravitational lens. The problem is that the field of view is tiny. We only get any useful ...
Steve Linton's user avatar
  • 10.3k
7 votes
Accepted

Algorithm to fit galaxies

The two most general, publicly available packages for galaxy image fitting (other than GALFIT) are probably Imfit and ProFit. (Note that I am the developer of Imfit.) There is also Lenstronomy, which ...
Peter Erwin's user avatar
  • 16.7k
7 votes

Is it possible to observe strong gravitational lensing with amateur telescopes?

If you have a good camera, a steady hand, good conditions and a lot of patience, you might. The Sunburst Arc, at a magnitude of around 17, has been observed with a 132mm refractor by Ian Woodward at ...
Thriveth's user avatar
  • 321
7 votes

Would the stars look different if many primordial black holes were present?

The answer is no, because these effects have been looked for and not found. The microlensing surveys of the 80s and 90s specifically set out to look for the lensing signatures of compact, massive ...
ProfRob's user avatar
  • 152k
7 votes
Accepted

Is quasar 2M1310-1714 outside the observable universe?

The redshift of the quasar is 1.975, so it is nowhere near the edge of the observable universe. 17 billion light years is the comoving distance (i.e. where it is now), as you can confirm with this ...
ProfRob's user avatar
  • 152k
7 votes
Accepted

Black hole magnification factor

I'd suggest you take a look at Narayan and Bartelmann (1996). They run through all the math to answer exactly this question. I'll add here the punchline from their paper for posterity. Shown below is ...
zephyr's user avatar
  • 15k
6 votes
Accepted

What does the 'amplification factor' of gravitational microlensing mean?

Gravitational lensing is just geometry, as is optical lensing. The amplification factor describes the increase of the area of the image (at constant surface brightness). BTW, the amplification ...
Walter's user avatar
  • 5,556

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