From nobody Thu Jan 29 13:39:42 1998 From: richmond@p674p06.isc.rit.edu (Stupendous Man) Newsgroups: sci.astro Subject: (Another reason) why astronomers prefer parsecs to light years Message-ID: X-Newsreader: slrn (0.9.3.2 UNIX) NNTP-Posting-Host: p674p06.isc.rit.edu Date: 29 Jan 98 17:51:45 GMT Organization: Rochester Institute of Technology Lines: 88 Path: newsfeed.cv.nrao.edu!newsgate.duke.edu!nntprelay.mathworks.com!news-peer.gip.net!news.gsl.net!gip.net!news-peer.sprintlink.net!news-backup-east.sprintlink.net!news-in-east.sprintlink.net!news.sprintlink.net!Sprint!129.21.3.100!isc-newsserver.isc.rit.edu!richmond Xref: newsfeed.cv.nrao.edu sci.astro:44698 Gene Nygaard asks angrily: > ... Why > don't you just use the SI units, metres, especially now that you have the > prefixes up through yotta- which are sufficient to express any distance > you could possibly measure with no more than 3 digits to the left of the > decimal point. > > Before you give me any b.s. about the convenience of inverting the > parallax angle in seconds to approximate this measure, remember that this > only works in a limited range. Yet astronomers use units such as > megaparsecs, which are never obtained by inverting a measurement of > parallax angle. I'll give you 4 reasons. A big reason which I'm sure has already been mentioned is: 1. because the technical literature has been using parsecs for many decades, and we want to continue to refer to it Gene Nygaard notes, correctly, that trigonometric parallaxes are measured only for a limited range of distances -- currently, out to about 300 parsecs. There are several proposed space interferometers which (if successful) will increase this by several orders of magnitude, but we still won't be measuring distances to the Virgo cluster via direct parallax. Nonetheless, 2. since the parsec _is_ a convenient unit for _some_ distance measurements, and we don't want to use two different units, we prefer to stick with it even at distances well beyond the reach of direct trigonometric parallax But the new twist I'll introduce is the following: astronomers measure the intrinsic luminosity of objects in "absolute magnitudes", which are defined as "the magnitude the object would have if it were at a distance of 10 parsecs." 3. if you replace the parsec, you must replace all "absolute magnitude" values, too Moreover, many observers use the unit "distance modulus" when discussing distances to extragalactic objects. If a star of KNOWN absolute magnitude M is _observed_ to have an apparent magnitude m, then the "distance modulus" is just that difference in apparent magnitude: (distance modulus) = (apparent mag) - (absolute mag B) For example, suppose you observe two Cepheid variable stars, each pulsing with a period of 100 days. From this identical period, you deduce that both stars have the same intrinsic luminosity, and hence the same absolute magnitude. Let's say it's M=-5. However, you observe the _apparent_ magnitudes of those stars to be m=15 and m=20. How far away is each? absolute mag apparent mag distance modulus distance ---------------------------------------------------------------------- -5 15 20 10 kpc -5 20 25 100 kpc Because of the way absolute magnitudes are defined, and the way distance modulus is defined, there is a simple, easy-to-remember relationship between the two *if the distance is expressed in parsecs*: (distance modulus/5) distance in parsecs = 10 Thus distance modulus (mag) 0 5 10 15 20 25 30 35 ---------------------------------------------------------------------------- distance in parsecs 10 100 1000 10^4 10^5 10^6 10^7 10^8 So my final rationale for keeping the parsec is because it meshes so well with absolute magnitudes: 4. if you replace the parsec, you lose the simple relationship between distance modulus and distance I maintain that astronomers should spend their time and energy on tasks other than replacing parsecs by light years, or by meters, or any other distance unit. Michael Richmond From nobody Thu Jan 29 15:25:04 1998 Path: newsfeed.cv.nrao.edu!newsgate.duke.edu!nntprelay.mathworks.com!news-peer-east.sprintlink.net!news-peer.sprintlink.net!news-backup-west.sprintlink.net!news-in-west.sprintlink.net!news.sprintlink.net!Sprint!207.91.5.5!news.navix.net!news From: "David W. Knisely" Newsgroups: sci.astro Subject: Re: (Another reason) why astronomers prefer parsecs to light years Date: Thu, 29 Jan 1998 19:43:23 +0000 Organization: Navix: Aliant Communications Internet Services Lines: 22 Message-ID: <34D0DBDB.2E6@navix.net> References: Reply-To: dk84538@navix.net NNTP-Posting-Host: cras9p2.navix.net Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 3.01 (Win16; U) To: Stupendous Man Xref: newsfeed.cv.nrao.edu sci.astro:44708 Hi there. Re: parsecs Vs. Light years. I think the debate on this is a little overblown. Many astronomers do use light years, usually when working with members of the general public or the press (very important in maintaining good relations and thus continued public money flow to observatories, and for research grants). The Light Year works somewhat better in the minds of many lay persons, since it is simply the distance light will travel in a year. Understanding the parsec is a bit much for many in the general public, which is why it isn't used with them. For most calculations, observations, and scientific publications, astronomers will use the parsec, and rightly so, since as you pointed out, the absolute magnitude is based on a 10 pc distance standard. Still, when it comes to dealing with those who have the purse strings, it might be nice to stick in the light year from time to time. Clear skies to you. -- David Knisely KA0CZC dk84538@navix.net Prairie Astronomy Club, Inc. http://www.4w.com/pac Hyde Memorial Observatory: http://www.blackstarpress.com/arin/hyde *********************************************** * Attend the 5th Annual NEBRASKA STAR PARTY * * July 18-25, 1998 http://www.4w.com/nsp * ***********************************************