# Tag Info

71

Sometimes we do, and the phenomenon is called a light echo. What you're looking at there is NOT moving gas. It's an "echo" exactly as you describe. The problem is that you need a pulse of light. If you have a constant stream of light, the "light echos" will be exactly like what you see in fog on earth.

52

I've made this into an answer because it's too long for a comment, and I really want to show the pictures. It is tempting to think of visible light as "close enough" to (near by wavelengths) and to conclude that "yes, actually, the yellow does affect it. I want a mirror without an obvious tint" However you are wrong, Physics will slap you down. Exhibit A ...

46

If you look at the reflectivity of gold (vs silver or aluminum) you can see a plateau at wavelengths below 500 nm source: If blue wavelengths are not reflected as well as other colors, the resulting image will look "more yellow" - which is what you see. At longer wavelengths, gold is a very good reflector (better than the other two above 600 nm). It also ...

24

Sort of. As Space.com writes, The raw Hubble images, as beamed down from the telescope itself, are black and white. But each image is captured using three different filters: red, green and blue. The Hubble imaging team combines those three images into one, in a Technicolor process pioneered in the 1930s. (The same process occurs in digital SLRs, except ...

24

It would be possible to see the progress of photons through space if the light pulse were exceedingly intense, and if the dust cloud from which they reflect were positioned and shaped to reflect the light toward us. Rather than shooting a beam from Point A to Point B, it would be better if the light source were between us and the dust cloud, as light ...

24

One cannot tell by the light spectra. Hydrogen and antihydrogen would give the same lines in the spectrum. The prevalence of matter over antimatter from other evidence indicates matter is predominant in the observable universe, and here is a nice review. How do we really know that the universe is not matter-antimatter symmetric? The Moon: Neil ...

17

You're right that the Sun being 4.5 billion years old makes observations difficult. The Sun goes around the Galaxy about once every 225 million years, so since the Sun formed it has gone around the Galaxy perhaps 20 times. The trouble is that the Galaxy is not like the Solar System: stars don't go around on nice nearly circular orbits, everything is a bit ...

15

Some numbers come from a review paper by Cullers (2000), who discusses the SETI Phoenix project. There, it is claimed that the Arecibo dish is capable of detecting a narrow band, coherent signal of $f=10^{-27}$ W/m$^2$ given a 1000 second observation. Assuming that this is an isotropic signal, then the implied power at distance $d$ is $p=4\pi d^2 f$, which ...

14

Martin Hoecker-Martinez's Answer is correct for perfectly noiseless observations of a two body Kepler system, i.e. the force between the bodies is directed along the vector linking them and the force magnitude follows and inverse square law with distance. An alternative to Martin's answer is that perfectly known position and velocity will determine all ...

12

If some of the light is reflected off the dust at such an angle that it is diverted to reach the observer, the observer will see that light. However, those specific photons reaching the observer will not reach B (unless they are reflected there by the observer). Similarly, unless the observer is at point B (which is not the case in the question as asked), ...

11

Generally speaking they refer to the distances from us when the light as emitted. No correction is usually made to say how far away the object is from us now, because this correction would be very small and inconsequential compared to the uncertainty in the original distance measurement. For instance, taking the Andromeda M31 galaxy as an example. Riess et ...

10

Using a camera that can capture "Motion at a Trillion Frames Per Second", this can be done at the laboratory scale. The technique used has been called femto-photography. (Image credit to Ramesh Raskar, Associate Professor, MIT Media Lab) Of course a camera that literally takes one trillion full frames per second is totally impossible with today's ...

10

An ideal Kepplerian orbit is defined by six (6) parameters: angular momentum (3) total energy (1) Laplace–Runge–Lenz vector which is perpendicular to angular momentum (2) Therefore you need at least six (6) independent observations. Astronomical observations are direction (but not range usually) given by a pair (2) of angles therefore three (3) ...

10

The classic color mapping for Hubble is described in Flase-color astrophotography explained. What you have is (in the Hubble palette): Line Freq True False Ha (656.3 nm) Red -- Green S-II (672.4 nm) Red -- Red O-III (500.7 nm) Green -- Blue An example of this for true color from John Nassr at Stardust Observatory at Coming to Life ...

8

Let's say you build a ping pong ball counter. It increments the count every time a ping pong ball hits the sensor. You throw a ball, and it hits the sensor: Detected! You throw a ball across the sensor from left to right... no detection, because you didn't hit the sensor. Your eyeball is a light sensor, which creates pictures from the light that hits ...

8

To ask what do physicists expect to accomplish with gravitational lensing is nowadays somewhat like asking what do biologists expect to accomplish with looking at things with microscopes. Gravitational lensing is a well established method used across astronomy and the main challenges the field itself has to tackle are mainly technicalities. But I will try ...

7

The atmosphere obscures data in three main ways; it absorbs light, it emits light in the infrared, and finally it diffracts light leading to distorted images. Observers have ways to deal with all three things, but I'll focus on the first two since they are more directly related to your question: 1) Atmospheric absorption. This plot gives a rough idea of ...

6

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 ...

6

Here is an update on (my) answer that your refer to. I have changed the visual threshold to V<6.5 mag (which is what Sky and Telescope have done) and I have used the revised Hipparcos reduction from van Leeuwen (2007) to obtain a (almost) complete catalogue of stars with their trigonometric parallaxes. It contains 7892 stars. I am not going to investigate ...

6

Yes, it's possible. Take a look at the diagram below. You're looking at the Earth from above the north pole. The yellow arrows indicate the direction of light coming from the Sun. The times on Earth show approximate local time; see how the Sun is most "direct" at noon but is below the horizon late in the night? (I think those times are most accurate during ...

6

The practical detection limit for HST is about a visual magnitude of 30 - that sort of number was reached in the ultra-deep field. Assuming that the solar sail kept reasonably stationary for the 100 hours or so of required exposure then we could do a calculation based on that. There is absolutely no need to resolve the object in order to detect it. If you ...

6

I find myself now answering my own question but only because the comment feature is not suited to this "comment". I have selected the answer by @HDE 226868 as my answer and primarily due to the linked Space.com reference. Very good answer to my question. In particular, I also thought this quote from the same page as being important as these reasons (below ...

5

A solid piece of metal with the thickness of 5 layers of foil is just like 5 layers of foil except that, since it is a single solid piece, the "layers" are able to conduct heat (quite quickly!) directly from one to the next. If the layers are instead separated by vacuum, the conduction of heat is eliminated, and heat transfer is greatly reduced (radiative ...

5

This paper contains an important analysis of the different trade-off between bandwidth and energy efficiency. The interesting conclusion from that paper is that the most energy-efficient way to send and receive interstellar messages (over flat spacetime) that maximise the bit-rate requires making the bandwidth of transmission very large. In particular, this ...

5

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 ...

5

Have a look at this article. It gives the number as $10^{24}$ rather than $10^{23}$, but it's such a vague estimate that a factor of ten is within the expected error. The number is the number of stars in the observable universe i.e. within 13.7 billion light years of Earth at the time the light we see today was emitted. Note that visible means visible to a ...

5

I did some research and I think I can answer my question myself now, after all. I hope you find it interesting. As it turns out, the technology of the Kepler space telescope would indeed allow detection of all Solar system planets except Mercury and probably Mars, i.e. all of them are big enough to be seen by it from a distance of about 2,000 ly. However, ...

5

The image is that of the entire sky above the telescope taken with some kind of fish-eye lens. It is quite common at observatories to use such arrangements to monitor for cloud cover. However you seem to have found a particularly poor example - possibly using a CCD imager with very few pixels. It is also possible that there is some light cirrus over the ...

5

The little $h$ is a historical artifact, one that will probably die out soon enough. The thing is, $H_0$ was extremely difficult to measure precisely for many decades after its importance was realized. At some point, cosmologists were divided between the "$H_0 = 50\ \mathrm{km/s/Mpc}$" and the "$H_0 = 100\ \mathrm{km/s/Mpc}$" camps. Because the quantity ...

5

Doubtful you'll find anything within the Solar System, but there are neutron stars, which are thought to have regions which are both superconducting and superfluid (that link is one of the original references from almost 50 years ago - there is a ton of literature on the topic since, you could start with some of these).

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