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Glossary term: Nebula

Description: A nebula is a distant celestial object that has the appearance of a cloud. Usually, a nebula consists of interstellar gas and dust. Historically, the term nebulae included any extended fuzzy-looking object, including what we today recognize as galaxies – distant star systems like our own Milky Way galaxy. Today, the term nebula is restricted to gas and dust clouds that are part of the interstellar medium – the gas and dust between stars within a galaxy. The category includes a number of different kinds of object: Molecular clouds are comparatively cold and dark, and predominantly consist of molecular hydrogen; inside clouds like this is where new stars are formed. Giant molecular clouds can contain up to a few millions of solar masses' worth of hydrogen gas. Young stars frequently emit narrow jets of ionized gas; when those jets excite the surrounding gas, the result is a type of nebula called a Herbig–Haro object. When massive stars have formed, their intense radiation makes the surrounding gas emit characteristic reddish light; the results are nebulae of hot and ionized hydrogen gas which are called HII regions. Other types of nebula are associated with the death of stars: low-mass stars leave behind expanding shells of gas that are (somewhat confusingly) called planetary nebulae. When a high-mass star explodes as a supernova, the ejected gas forms a type of nebula called a supernova remnant.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Diagrams


Sagittarius is shaped like a teapot pouring tea south west. The ecliptic runs WSW to ENE at the top of the constellation

Sagittarius Constellation Map

Caption: The constellation Sagittarius along with its bright stars and surrounding constellations. Sagittarius is surrounded by (going clockwise from the top) Aquila, Scutum, Serpens Cauda, Ophiuchus, Scorpius, Corona Australis, Telescopium, Microscopium and Capricornus. The brighter stars in Sagittarius form a distinctive teapot shape. Sagittarius lies on the ecliptic (shown here as a blue line), this is the path the Sun appears to take across the sky over the course of a year. The Sun is in Sagittarius from mid December to mid January. The other planets of the Solar System can often be found in Sagittarius. Sagittarius lies south of the celestial equator. The famous teapot asterism is visible for all but the most arctic regions of the world but the most southerly parts of the constellation are not visible in northern parts of Asia, Europe and North America. Sagittarius is most visible in the evenings in the northern hemisphere summer and southern hemisphere winter. The supermassive black hole Sagittarius A* which lies at the center of our Milky Way Galaxy is sits on the western (here right-hand) edge of Sagittarius. Due to it covering an area at the center of our Galaxy, Sagittarius is home to many star clusters including open clusters (marked here with yellow circles) and globular clusters (marked here with yellow circles with + signs superimposed on them). Three nebulae are also marked here with green squares. The y-axis of this diagram is in degrees of declination and with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Related Activities


Reading the Rainbow

Reading the Rainbow

astroEDU educational activity (links to astroEDU website)
Description: By understanding how rainbows work, you can discover about light and its properties, learning about stars, nebulae, galaxies, and our Universe.

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

Age Ranges: 14-16 , 16-19 , 19+
Education Level: Informal , Middle School , Secondary , University
Areas of Learning: Interactive Lecture , Observation based , Social Research
Costs: Low Cost
Duration: 1 hour 30 mins
Group Size: Group
Skills: Analysing and interpreting data , Asking questions , Engaging in argument from evidence