Questions tagged [units]

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5
votes
3answers
245 views

What does BJD-2450000 mean?

In different papers I've encountered time indications in BJD-2450000. I know that BJD stands for Barycentric Julian Date, but I don't understand the meaning of the suffix -2450000. This is an example ...
1
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1answer
31 views

How to use spectral flux density data while using Planck's law giving spectral radiance?

I'm currently working on a fun project in my free time where I'm trying to calculate the temperature, among other things, of stars based on their spectra. Since I have essentially no prior experience ...
4
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1answer
58 views

Flux density from brightness map

This is a naive question. I have a temperature map of radio emission at 1420 MHz, in Kelvin. I want to extract the flux density (in Janskys) from an extended region of this map. Is the flux density ($...
3
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1answer
74 views

'Little h' usage in cosmological simulations

I am running a cosmological simulation and am having some trouble putting things into code units. The physical distance units in my simulation are in terms of $\text{Mpc/h}$, where $h$ is the ...
1
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0answers
73 views

Units confusion while calculating flux of a star using distance and luminosity, Python

I am trying to calculate the flux of various stars in Python. I am using the information from a database containing all stars in Hipparcos, Yale Bright Star, and Gliese catalogs (almost 120,000 stars)....
0
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1answer
40 views

Correct units to use while plotting mollweide projection of sky chart with RA and Dec values, matplotlib

I am trying to plot a mollweide projection of a sky chart in Python using the matplotlib package. I am using the RA and Dec values from a database containing all stars in Hipparcos, Yale Bright Star, ...
1
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0answers
30 views

How to convert from Black body intensity to MJy/sr?

How does one convert from $ergs\cdot cm^{-2}\cdot s^{-1} \cdot sr^{-1}\cdot Hz^{-1}$ to $MJy \cdot sr^{-1}$ and from $ergs\cdot cm^{-2}\cdot s^{-1} \cdot sr^{-1}\cdot cm^{-1}$ to $MJy \cdot sr^{-1}$...
2
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2answers
80 views

How to convert apparent visual magnitude to SI units?

How do I convert the apparent visual magnitude of astronomical objects to SI units (lumens)?
2
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1answer
70 views

How to convert from FELO-HEL to Km/s?

I am using DS9 to calculate the rotational velocity of a galaxy. I have 21cm cube and I used DS9 to plot this graph I do not understand what is FELO-HEL! and I want to convert the units from FELO-...
1
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1answer
74 views

What is “g-98” in plots of stellar spectra?

I was looking to find some example carbon star spectra and came across this page on the Harvard website which shows several plots of carbon star spectra. The horizontal axis is labelled as wavelength ...
1
vote
1answer
131 views

What does it mean when there are two plus-or-minus signs for a measurement?

As far as I know, the plus-or-minus sign is used to indicate the statistical margin of error of a measurement. But what does it mean when there are two plus-or-minus signs in the same measurement? ...
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0answers
174 views

Conversion factors for flux

I have a flux value in the infrared band of 8 mJy at a wavelength of 80 microns that I want to translate into X-ray conventional ...
11
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1answer
2k views

A “strange” unit radio astronomy

I'm reading up on radio astronomy, and I came across this paper from 1964. At the bottom of page 193, the author uses a unit that I've not seen before in discussing radio power emission from stars: ...
2
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1answer
84 views

Units in optical and radio spectral data cubes: Flux vs Brightness

I have noticed that the units used in spectral data cubes coming from optical IFU instruments are usually in $erg/s/cm^2/Ang/pixel$. To my understanding this is a measure of Flux. Right? However, in ...
4
votes
2answers
2k views

Converting Jy/beam to Jy?

Maybe its a dumb question, but to convert Jy/beam to Jy, I just have to multiply it by the beam size in sr right? Being $\Omega$ the beam size: $\Omega = \frac{\pi \theta_{maj} ~~ \theta_{min}}{4 \...
3
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2answers
475 views

Finding the radius of a star in arcsec

If i know the radius r of a star in cm, how can I convert it to arcseconds? For example, if I have a star with r = 3.18e13 cm, and distance to the star d = 220 parsecs, what is the relation to ...
2
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1answer
80 views

Units of integrated flux density (irradiance) along longitudes

If I've taken an image of the Galactic bulge with the pixel units in MJ per steradian: and I've looked down each column of the image and added up the values: then do the units change? My thinking is ...
1
vote
1answer
74 views

Natural units of star clusters angular momentum and energy

I'm calculating the total angular momentum of a bunch of theoretical stars of a few solar masses, separated by a few light years, moving at velocities of about a few dozens of km / sec. Mechanical ...
1
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1answer
1k views

How are people converting intensities in Janskys to Kelvin?

I'm reading some ALMA proposals and I am often seeing a conversion from Jy to Kelvin when, for example, people quote noise levels or source flux. For example on their sensitivity calculator (https://...
77
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13answers
19k views

Why don't astronomers use meters to measure astronomical distances?

In astronomy distances are generally expressed in non-metric units like: light-years, astronomical units (AU), parsecs, etc. Why don't they use meters (or multiples thereof) to measure distances, as ...
2
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1answer
129 views

wavelength-based IFU data cubes VS velocity-based IFU data cubes

I have 2 simulated IFU data cubes of the same observation, let's say A and B. The 3rd dimension of cube A is in wavelengths. In order to create cube B I just rebinned cube A in log-wavelengths and ...
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0answers
99 views

${\rm W/m}^2$ to Jansky [duplicate]

Lets assume I have a source and receive $8 \times 10^{-20}$ ${\rm Wm}^{-2}$ in (400-750) nm filter. How do you convert this received flux to the unit Jansky, is there a formula? (Ultimately, I want to ...
3
votes
1answer
2k views

Why levels of radio contours maps are given in mJy/beam and what does it mean?

I know what a jansky is, but I don't understand what beam (in $\frac{1 Jy}{beam}$) is. If jansky is a unit of flux density, what kind of unit is $\frac{1 Jy}{beam}$?
3
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2answers
2k views

Does the age of the universe take into account General Relativity / Special Relativity?

It is generally accepted that the age of the universe is approximately 12-15 billion years old based on the speed of the expansion of the universe. Since everything is moving very fast away from us, ...
1
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1answer
790 views

Converting ångström spectral dimension to galaxy speed (km/s)

I have a spectral cube (in FITS format) whose spectral dimension is in ångströms. The sampling along the spectral dimension is 0.28A (CDELT=0.28). The observation in the cube is Ha emission of a ...
16
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3answers
3k views

Is the Earth-Sun distance 1.012 AU?

Wolfram Alpha's query: Sun Earth distance in AU gives: 1.012 au (astronomical units) They definitely use this value in other places too - e.g. Earth-Sun $L_1$ distances from Earth and from Sun ...
3
votes
1answer
103 views

Astrophysical “unitised” version of the Gravitational constant

This question relates formalising the gravitational constant, $G$, (which in S.I. units is usually quoted as $\sim6.67\times10^{-11}\,\mathrm{m}^{3}\,\mathrm{kg}^{-1}\,\mathrm{s}^{-2}$) in units which ...
2
votes
2answers
623 views

What is projected separation and how can I make sense of its unit (h^-1 kpc)?

I have been reading this paper on the separation of a binary quasar in which it states: At a projected separation of 8.3/h kpc (Omega_matter = 1), CTQ 839 would be the smallest projected separation ...
2
votes
2answers
739 views

What are the units of distance in this subtended angle calculation?

I'd like to find the distance at which a 2.4 cm coin is subtended by an angle of 10". I've drawn my diagram and found that $D=\dfrac{d}{\alpha}$ using the small angle approximation. Since $10" = 4.85\...
2
votes
1answer
92 views

How often and over what period is Earth’s rotation averaged to compute UT1?

I understand that UT1 (and for that matter UT0, UT2, etc.) are based on averages of actual earth rotation, and serve as a form of mean solar time. However it’s not clear to me when these averages are ...
1
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2answers
84 views

What are some applicable problems with the correct usage of G?

In some fields of astrophysics, where distances are measured in parsecs (pc), velocities in kilometers per second (km/s) and masses in solar units (), it is useful to express G as approximately 4.302 ...
2
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2answers
73 views

What is frequency range?

Using the NED, you can search via frequency range corresponding to the selected pass band filter. I'm a beginner here and don't really understand. Here's the quote: NOTES: Objects with observations ...
1
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3answers
234 views

Basic celestial data

complete beginner here, only got into star gazing a few days ago using my friend's telescope and I don't really understand at all what any of the measurements mean. Anyway, talk about jumping in the ...
5
votes
2answers
129 views

apparent size of an astronomical body

What's the name of the measure of apparent (visible) size of a body? e.g. the Sun and the Moon appear to be roughly the same size, despite the Sun being vastly bigger - because the Moon is much closer....