Beginners - nebulae

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" colspan=2 height=22 BGCOLOR="#330099" TEXT="#080000" bgproperties="fixed" background="040000d2erkqrf.gif"> <div> Derby and District Astronomical Society - Aries Magazine</div> <div> </div> </td> </tr><tr> <td valign="top" width=145 BGCOLOR="#333399" TEXT="#080000" background="03fa2c00erkjzj.gif"> <div> <A HREF="index.htm" TARGET="_top" TITLE="Aries Astronomy magazine on line Derby a"><U>HOME</U></A></div> <bR> <div> <A HREF="editorial.htm" TARGET="_top" TITLE="Aries on line Autumn 2000 editorial"><U>Aries on line - Autumn 2000 editorial</U></A></div> <div> <A HREF="double_stars.htm" TARGET="_top" TITLE="Double Stars"><U>Double Stars</U></A></div> <div> <A HREF="id25.htm" TARGET="_top" TITLE="Pluto and Back for bed"><U>Pluto and Back for bed</U></A></div> <div> <A HREF="vla.htm" TARGET="_top" TITLE="VLA Radio telescope"><U>VLA Radio telescope</U></A></div> <div> <B>Beginners - nebulae</B></div> <div> <A HREF="id28.htm" TARGET="_top" TITLE="Forgotten Astronauts"><U>Forgotten Astronauts</U></A></div> <div> <A HREF="id29.htm" TARGET="_top" TITLE="Sky Scene Oct Dec 2000"><U>Sky Scene Oct - Dec 2000</U></A></div> <bR> </td> <td valign="top" height=458 BGCOLOR="#FFFFFF" TEXT="#080000" background="06a92963.gif"> <div> <I>Aries Astronomy magazine on line- Derby and District Astronomical Society</I></div> <div> <IMG SRC="04107040.gif" border=0 width="263" height="4" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> Beginners - Nebulae.</div> <div> <I><B>Author: Kevin Woodward</B></I></div> <bR> <bR> <div> In this article I hope to explore the objects called nebulae. The word nebula simply means “cloud”. Before the advent of large telescopes, many galaxies were incorrectly called nebulae. This was because observers just could not see the individual stars that made up the galaxies. A true nebula is a cloud of dust and gas. Some are bright whilst others are dark. Some are areas where new stars are being formed. Some are the remnants of the death of stars. </div> <bR> <div> Bright nebulae.</div> <bR> <div> <IMG SRC="08b6e6b0.jpg" border=0 width="110" height="107" ALIGN="BOTTOM" HSPACE="0" VSPACE="0">  The Trifid nebula - M20 (courtesy of <A HREF="http://oposite.stsci.edu/pubinfo/Pictures.html" TARGET="_top" TITLE="http://oposite.stsci.edu/pubinfo/Picture"><U>NASA</U></A> site)</div> <bR> <div> Some are areas where new stars are being born. Good examples are the Orion nebula and the Trifid nebula. In these, gravitational forces are causing the gases (mainly hydrogen) and dust to come together. Over millions of years, the gases are compressed by  gravitational forces in the nebula. This compression causes the gas and dust to heat up. The scale is so large that the temperature becomes high enough to start nuclear reactions at the centre of the gas/dust cloud – a new star is born. The light from the new star then illuminates the surrounding gas and dust, rather like a lamp. This enables us to see the nebula. These are  called reflection nebulae. In some cases, the radiation from the new star (or stars, as the case may be) is so intense, it makes the gas glow (rather like the way that a fluorescent tube works). These are called emission nebulae.</div> <div> The other types of bright nebulae represent the death of a star. The ultimate fate of a star depends upon its initial mass. If it is greater than 1.4 times the mass of the Sun, it is likely to end with a huge explosion known as a supernova. In this explosion, much of the mass of the star is ejected into space. This travels out into the surroundings at high speed as a cloud. The core of the star ends up a a very dense object. </div> <bR> <div> <IMG SRC="08e787a0.jpg" border=0 width="120" height="122" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"> M57, a planetary nebula  (courtesy of <A HREF="http://oposite.stsci.edu/pubinfo/Pictures.html" TARGET="_top" TITLE="http://oposite.stsci.edu/pubinfo/Picture"><U>NASA</U></A> site)  </div> <bR> <div> Any light or other radiation that is given off by this can illuminate the expanding gas cloud. The classic examples of supernova nebulae are the Crab nebula (M1 – the first on Messier’s great list) in Taurus and the veil nebula in Cygnus. Stars of between 0.4 and 1,4 times the mass of the Sun end their lives more gently. After their hydrogen fuel in the core is used up, they cool down and swell in size, becoming red giants. This is as far as things can go in terms of nuclear reactions in such stars. After the rest of the hydrogen is used up, the outer layers of the star are puffed off into space, forming an expanding cloud of gas. The rest of the star becomes a white dwarf star. The light from this illuminates the expanding gas cloud and from Earth we see a planetary nebula. The shape that we see depends on factors such as the density of gas and the brightness of the central star. </div> <bR> <div> Dark nebulae.</div> <bR> <div> These are caused where there is no star to illuminate the gas and dust so they block out the light from more distant stars. William Herschel commented on dark nebulae as being “holes in the sky”, since they do give that appearance. In the illustration of the Trifid nebula, the three dark areas are areas of dust and gas that are not illuminated. The so called coal sack nebulae of the milky way (areas in which the milky way stars seem to be less dense) are dark nebulae. When you next look at the milky way, look for a number of these areas in the constellation of Cygnus. The famous Horsehead nebula in Orion is a dark nebula that gives the appearance of the head of a seahorse. </div> <bR> <div> <IMG SRC="096b4e10.jpg" border=0 width="180" height="225" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> If you have any questions about the information in this article, let me know and I will try to explain in more detail. Any questions could form the basis of a future section of the magazine. </div> <bR> <div> <A HREF="http://oposite.stsci.edu/pubinfo/Pictures.html" TARGET="_top" TITLE="http://oposite.stsci.edu/pubinfo/Picture"><U>Visit the NASA site for plenty more stunning images of nebulae.</U></A></div> </td> </tr></table></body>