# All Questions

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316 views

### Shouldn't we not be able to see some black holes?

Whenever there is a picture of a black hole, (even though they are artist depictions, this is how they are described) most are shown kind of in a flat disk like this. My question is, why do we see ...
2 views

### Get a table of moon phase angle per day

Where can I get a table with the moon phase angle for each day ? It's availble here for a given day, but I'd have to input each day of a year to get a full table.
53 views

### What is a black hole's “shadow” and the best angle to view it to measure the spin?

This excellent answer mentions several ways to try to observationally measure the spin of a rotating black hole. The third one is intriguing, but I don't understand how this works: The spin of a ...
11 views

### How to model and subtract nebular emission lines from stellar spectrum

I have optical stellar spectra from a star forming region with spatially varying nebular background. I want to subtract the nebular lines from the stellar spectra (which are varying in intensity from ...
902 views

### Is it possible that the shadow of the moon is a single dot during solar eclipse?

During a solar eclipse the moon covers the sun on some places on the Earth. Now the focal point is a bit above the Earth so the shadow area is about 160 miles. In history the moon was a bit closer to ...
28 views

### How much volume of gases could be added to Mars atmosphere that the planet gravity can retain?

This article Mars atmosphere states that Today Mars atmosphere is very thin, but it was not always like this. Around 3.5 billion years ago, the red planet had a thick enough atmosphere for liquid ...
99 views

### Is every star formed in an open star cluster?

As far as I understand, an open cluster is formed from a single molecular cloud, with each star in the cluster having roughly similar age and properties. Our Sun is not part of any star cluster, but ...
66 views

### The dominance of Gravity vs Strength and its dependency on crater size

So I am trying to write a research paper (about 3700 words) for high school about the topic in the title or something similar. I have found the following information which I am struggling to ...
29 views

### Formation of the First Stars

I've got a few questions about the first stars to form in the universe. First off how might metalicity have impacted the formation of the first stars and also what effect would the absence of metals ...
44 views

### Cosmological redshift vs doppler redshift

I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift. He ...
132 views

### Can we see evidence of sunspots on other stars

Searching for exoplanets by measuring the change of a stars brightness when an exoplanet transits the star's disk also has the potential to detect exo-sunspots. Have they ever been detected?
3k views

### How do we know that black holes are spinning?

How is it possible to know if a black hole is spinning or not? If a planet is spinning, you can see it clearly but you can't really see a black hole. Next thing would be that matter interacts with ...
133 views

### When did people discover that the Earth is not an ideal sphere, but a little flattened?

When did people discover that the Earth is not an ideal sphere, but a little flattened? And if you know, who was the first person to discover it?
117 views

### What is the size of the largest asteroid orbiting the Earth, other than the Moon?

It seems odd to me that no other asteroid around the Earth is even close to the size of the Moon. Anyone have any idea about the size and orbit radius of Earth's "second moon" ?
879 views

### Is it possible to use Hubble Telescope to observe Earth in the past?

We can observe many astronomical objects and structures beyond our solar system, however our observations are as the objects appeared in the past due to the speed of light. My question: "Is it ...
33 views

### Estimate prevalence of ʻOumuamua-style interstellar obects inside Jupiter orbit now that C/2019 Q4 (Borisov) is found?

Now we know two of them, so maybe it's not an outlier. We have two points so we can draw a line. How many similar macroscopic objects should be zipping through space at ~30 km/sec so that we would ...
15 views

### Dispersion: any software to work out the dispersed waveforms of EMW signals that propagate via interstellar medium for some distance?

Is there any software (toolbox, code, package, paid or opensource are both ok), to work out the dispersed waveforms of EMW signals (time series data are given), that propagate via the interstellar ...
28 views

52 views

### Sky from Equator at 8,000 years Before Present?

The Stellarium software allows future views of the sky. I need an online resource (planetarium projector?) that shows pre-historical views, and from the Equator at 8,000 years before the present.
75 views

### What makes scientists already so certain that Comet C/2019 Q4 (Borisov) is a comet and not an asteroid?

US Naval Research Laboratory scientist Dr. Karl Battams has recently tweeted Unlike 'Oumuamua, whose asteroid-or-comet nature still gets debated, this one is definitely a comet. If it is ...
39 views

### Does the Sun have a numerical designation in any star catalogues?

Given the existence of large star catalogues listing millions of astronomical objects with numerical designations (such as Tabby's Star, KIC 8462852) it occurred to me that one of them might contain ...
33 views

### Any night sky image databases

I was wondering if there were any available databases for photos of the night sky. It doesn't have to feature pictures of any particular star, just has to be a large collection of photos from the ...
546 views

### Can I see Harvest moon in India?

I just saw articles which say the Harvest moon will be visible in the US. Apart from the US, in which other country can people see it? Can I see it from India?
62 views

### Storm on Earth vs. storm on Jupiter?

Why is the storm on Jupiter much stronger than storms on Earth and why don't the storms on Earth get as big as Jupiter's great red spot
24k views

### Why did Venus not lose its atmosphere without magnetic field?

It is often stated that the magnetosphere not only shields the planet from cosmic radiation, but also prevents atmospheric loss. Why then did Venus not lose most of its atmosphere if it doesn't have a ...
17 views

### Redshift, velocity, distance

How are redshift, (recessional) velocity and distance of a galaxy related? I understand what v=HD states. So I assume that Hubble measured a direct relation between the distance and recessional ...
54 views

### Existence of a mass-volume astronomical chart?

I've been wanting to make a mass-volume (or mass-radius) chart for astronomical objects ranging from asteroids to supermassive black holes on a log-log scale. A sort of 'Hertzsprung-Russel' type ...
33 views

### Why does the outside edge of a Kerr black hole ergosphere rotate at the same speed as the inner edge?

Reading through the Wikipedia on a Kerr black hole ergosphere, and this nice question on the maximal spin rate of a black hole leads me to ask: why does the outer edge of the ergosphere rotate at the ...
40 views

### What is the intensity distribution of visible light over the solar disk?

What is the distribution of the visible spectrum over the solar disk? Images of the Sun using the visible spectrum suggest that the light emmitted from the edge is almost equal to the light emmited ...
55 views

### How to derive the date (day, month) from the day of year? [on hold]

Is there any formula to derive date from day of year? Say I want to know the date of 255th day of 2019. That is 9/12/2019.
106 views

### Size of the earth from the moon [duplicate]

I was hoping to get some serious responses to the effect of, as an example, the earth’s atmosphere being very wet, containing many water droplets can act as a magnifying glass. Were it not so the moon ...
78 views

### Would two Earth-like planets on opposite sides of a Sol-like star at about 1 AU be a stable configuration?

Would two Earth-like planets on opposite sides of a Sol-like star at about 1 AU be a relatively stable configuration? I understand that there are tadpole-like orbits and the two planets may not stay ...
889 views

### antimatter annihilation in stars

Electrons annihilate with positrons produced through fusion processes in stars. Which particle interaction produces new electrons so that the sun isn't deplete of electrons? Or is something else ...
3k views

### Calculate Day of the Year for a given date

I am trying to make sense of the following formula that claims to calculate the day of the year given today's date represented by (day/month/year): ...
101 views

### Why is this telescope so short? How hard is it to make such a fast primary?

The recently discovered object C/2019 Q4 (Borisov) is in the news because it might have an eccentricity greater than one, which means it might be from outside the solar system, though not necessarily. ...
91 views
+100

### Any simple experiment with an H-alpha telescope?

Is there any simple experiment that can be done with an H-alpha telescope, for example to estimate the Sun's mass, size, distance, temperature or intensity? I give lessons about renewable energy and ...
34 views

### Gravitational lensing

I was reading about gravitational lensing. I was just curious to know if there are any formulae or methods to find the angle at which the light deflects due to strong gravitational lensing? And how ...
35 views
+100

### What are realistic and unrealistic values for the high energy gamma ray spectral indices of pulsar wind nebulae?

This summer, I was working on a project fitting very high energy gamma ray spectra of the Crab Nebula, a pulsar wind nebula. At energies about $\sim$1 TeV, a simple power law suffices, i.e. \$\phi(E)\...
522 views

### I need help using this telescope!

The telescope I'm using is a National Geographic 76 Diameter, 700 focal length, Equatorial Mount. It's hard to use the equatorial mount, as this is my first telescope it's hard to use. Everything I ...
328 views

### How is it ensured that a parabolic shape forms while polishing a telescope mirror?

I understand polishing the mirror with a successively finer abrasive can create a curvature, but how is it possible to obtain a uniform curve that forms a parabola while polishing by hand?
83 views

### Where would an object fly to if it were to leave the gravitational field of Earth? And would it stop somewhere? What would happen to it?

My 3,5 year old son is asking :-) He can take answers for a 15 yo I think. Many thanks for your help!
9k views

### Is there a star over my head?

Say I'm standing up straight, and I draw a straight line from my core through the top of my head (perpendicular to the ground). What is the probability that that line intersects with a star? EDIT: I'...
361 views

### How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

The ESA video ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole includes a clip of images of stars at the center of our galaxy orbiting around SgrA*, a ...
25 views

### What equipment do I need to measure the spectrum of a star?

For an amateur astronomer, is it possible to measure the spectrum of a star? I can easily take a photo of a star just by putting a camera on one end of a telescope and over exposing it. But I would ...
21 views

### Angle of the Sun during winter solstice

What is the angle of the Sun on the eastern horizon at 35° latitude if it is the winter solstice?
156 views

### How can we detect water on Mars-like exoplanets?

According to data from Curiosity, Mars' dust holds about 2% water by weight This wasn't previously detected, so the impression we have had of Mars being incredibly dry may need to be altered. Okay it ...
34k views

### How does neutron star collapse into black hole?

We know the spectacular explosions of supernovae, that when heavy enough, form black holes. The explosive emission of both electromagnetic radiation and massive amounts of matter is clearly observable ...
77 views
+50

### How does the orbit of a 2-body system evolve as it orbits a more massive 3rd body?

I have been reading up on how multiplicity of star systems decline with star mass, which implies a relatively common occurrence of binary star systems orbiting more massive stars. I would imagine this ...