Astronomy

Astronomy

Frequently Asked Questions (FAQ)   

Refer to brief answers to the most Frequently Asked Questions on General Astronomy, Astronomical Instrumentation and Computing to help you find what you want faster, with source websites for additional information if required.  See also COSMOS - The SAO Encyclopedia of Astronomy.  If you have any questions for our team or comments, please contact us

What is apparent and absolute magnitude?

What is parallax and proper motion?

What does an H-R diagram show?

What does the letter M stand for when referring to an astronomical object?

What do the letters NGC stand for?

What is Electromagnetic Radiation, Spectroscopy and Spectometry?

What is a CCD?

What does a Telescope do?

What are bmp, gif, jpg, fit and tiff files?

What is a PDF file and how can it be accessed?

What is a 'Zipped' file and how can it be 'Unzipped'?

How to Unzip a compressed file?

General Astronomy

What is apparent and absolute magnitude?

Magnitude is a measure of brightness on a logarithmic scale.  The apparent magnitude (m) of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere.  Absolute magnitude (M) is defined as the magnitude a star (or any other celestial object) would have if placed at an exact distance of 10 parsecs (32.6 light years) from us. (Wikipedia) The 'distance modulus' is the difference between the apparent magnitude and absolute magnitude of a celestial object (m - M), and provides a measure of the distance to the object, r.

Further information on apparent magnitude and absolute magnitude can be viewed at COSMOS

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What is parallax and proper motion?

Parallax refers to the apparent displacement of an object as seen from two different points that are not on a line with the object and is measured by the angle or semi-angle of inclination between those two lines.  View an online applet.

The proper motion of a star is its angular change in position over time as seen from the Sun and is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree (Wikipedia) 

Further information on parallax and proper motion can be viewed at COSMOS

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What does an H-R Diagram show?

Messier_objects_table

The Hertzsprung-Russell diagram plots the temperature of stars against their luminosity (the theoretical H-R diagram), or the colour of stars (or spectral type) against their absolute magnitude (the observational H-R diagram, also known as a colour-magnitude diagram).  Create your own Colour Magnitude Diagram using our Star Clusters Photometry module.

Further information on the H-R diagram at COSMOS

Image credit: R. Hollow, CSIRO

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What does the letter M stand for when referring to an astronomical object?

Messier_objects_table

The Messier objects are those catalogued by Charles Messier in "Catalogue des Nébuleuses et des Amas d'Étoiles" (Catalogue of Nebulae and Star Clusters) and many are still referenced by their Messier number.  The first edition included 45 objects with Messier's final list totaling 103. Other astronomers filled out the list to 110 using side notes in Messier's texts. (Image and text credit Wikipedia)

NB: Most of these objects are also classified with an NGC reference following the Messier reference.

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What do the letters NGC stand for?

LCOGTN_space_images

New General Catalogue - In full, the New General Catalogue of Nebulae and Star Clusters, the NGC is a catalogue of nearly 8,000 nebulae, star clusters and galaxies. It was originally published in 1888 and later expanded by the publication of the IC or Index Catalogue.  (Credit ESky)

Space Images, courtesy LCOGTN

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What is Electromagnetic Radiation, Spectroscopy and Spectrometry?

ElectromagneticWavesDiagram 

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Electromagnetic radiation (often abbreviated E-M radiation or EMR) is a phenomenon that takes the form of self-propagating waves in a vacuum or in matter.  It comprises electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation.  Electromagnetic radiation is classified into several types according to the frequency of its wave; including (in order of increasing frequency and decreasing wavelength): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Wikipedia
 

Spectroscopy - originally study of interaction between radiation and matter as a function of wavelength (λ), it later expanded to measurement of quantity as function of wavelength or frequency, and then added energy (E) as a variable. A plot of the response as a function of wavelength/frequency is a spectrum.  Spectrometry is the technique used to assess the concentration or amount, and the instrument is a spectrometer/spectrograph. Most large telescopes use spectrometers to measure the chemical composition and physical properties of astronomical objects or their velocities from the Doppler shift of their spectral lines. (Image and text credit Wikipedia)

Further information on electromagnetic radiation and spectroscopy can be viewed at COSMOS

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Astronomical Instrumentation

What does a telescope do?

A telescope is usually used to focus electromagnetic radiation so that astronomers can observe distant sources by using large effective areas to collect the most light

Further information on telescopes can be viewed at COSMOS

What is a CCD?

CCD_image_sensor_on_flexible_circuit

A charge-coupled device (CCD) is a light-sensitive integrated circuit that stores and displays the data for an image so each pixel (picture element) in the image is converted into an electrical charge the intensity of which is related to a color in the color spectrum.  (Text credit Tech Targets), Image Wikipedia

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Computing

What are bmp, gif, jpg, fit and tiff files?

BMP_logo

BMP (Bitmap image) can be pronounced 'bump'.  The BMP format is a commonly used raster graphic for saving image files and stores colour data for each pixel in the image without any compression. eg a 10x10 pixel BMP images will include colour date for 100 pixels.  This method of storing image information allows for crisp, high-quality graphics but also produces large files sizes.  (Credit TechTerms)

FIT_logo

FITS (Flexible Image Transport System - FIT) is a data format designed to provide a means for convenient exchange of astronomical data between installations whose standard internal formats and hardware differ. A FITS data file is composed of a sequence of Header Data Units (HDUs).  The "Image" in FITS comes from the original use of the format to transport digital images, but not any more.  (Credit Coverpages)

Jpeg_logo

GIF (Graphic Interchange Formats) and JPEG (Joint Photographic Experts Group jpg, jpeg) and files are also known as bitmaps, but use image compression algorithms that can significantly decrease their file size.  For this reason, jpeg and gif images are used on the Web, while BMP images are often used for printable images.  (Credit TechTerms)

TIFF (Tagged Image File Format - TIF, TIFF) is the graphics file format created in the 1980's to be the standard image format across multiple computer platforms. The TIFF format can handle color depths ranging from 1 to 24-bit. There are around 50 variations of the TIFF format. Recently, JPEG has become the most popular, due to its small file size and Internet compatibility. (Credit TechTerms)

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What is a PDF file and how can it be accessed?

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PDF (Portable Document Format) is a file format developed by Adobe Systems. PDF captures formatting information from a variety of desktop publishing applications, making it possible to send formatted documents and have them appear on the recipient's monitor or printer as they were intended. (Credit Webopedia)

To view a file in PDF format you need Adobe Reader or another PDF reader utility.

What is a 'Zipped' file and how can it be 'Unzipped'?

Zip_logo

The ZIP (zip, ZIP) file format is a data compression and archive format. A ZIP file contains one or more files that have been compressed to reduce file size, or stored as-is. The ZIP file format permits a number of compression algorithms but, as of 2009[update], the Deflate method continues to be dominant. This algorithm is used by a range of applications such as WinZip and Stuffit Expander and other utilities. (Credit Wikipedia)

How to Unzip a compressed file refer to options here


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