Spica 15th brightest star??? no way

[buckeye101]

I constantly look at Spica and wonder how it could be the 15 or 16th brightest star in the night sky according to several websites. then i notice Arcturus and its supposed to be the 4th brightest, when vega easily outshines it and Spica outshines them both times 2! Who's coming up with this, and why is Spica the 15th brightest (supposedly) when to my eyes it is easily the brightest star times 2 out in the night sky???

 

[buckeye101]

I hope Im looking at Spica and not venus <img src="/images/icons/wink.gif" /> But i followed the arc (of the dipper) to Arcturus and then "sped" on to Spica <img src="/images/icons/smile.gif" />

 

[tfwthom]

There are two scales apparent and absolute magnitudes.<br /><br />Apparent is how bright the appear to us in the sky. The scale is somewhat arbitrary, but a magnitude difference of 5 has been set to exactly a factor of 100 in intensity. As it turns out, the eye senses brightness logarithmically, so each increase in 5 magnitudes corresponds to a decrease in brightness by a factor 100<br /><br />Absolute magnitudes are how bright a star would appear from some standard distance, arbitrarily set as 10 parsecs or about 32.6 light years. Stars can be as bright as absolute magnitude -8 and as faint as absolute magnitude +16 or fainter.<br /><br />There are thus (a very few) stars more than 100 times brighter than Sirius, while hardly any are known fainter than Wolf 356. <br /><br /><br />The magnitude scale was invented by an ancient Greek astronomer named Hipparchus in about 150 B.C. He ranked the stars he could see in terms of their brightness, with 1 representing the brightest down to 6 representing the faintest. Modern astronomy has extended this system to stars brighter than Hipparchus' 1st magnitude stars and ones much, much fainter than 6.<br /> <br /><br />List of the brightest stars http://www.seds.org/Maps/Stars_en/ <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>

 

[buckeye101]

whoa! I'm sorry, but can you dumb your response down a bit for me??? Thanks

 

[tfwthom]

Let's try this....<br /><br />There are two scales apparent and absolute magnitudes. <br /><br />Apparent is how bright they appear to us in the sky. This also has to do with how far a star is away from us. Sirius 8.6 light years away mag -1.46 is closer then Spica 220 light years away mag 0.98 so looks brighter. Your location on the planet will also make a difference.<br /><br />Absolute magnitudes are how bright a star would appear from some standard distance, arbitrarily set as 10 parsecs or about 32.6 light years. So Spica at mag -3.2 is brighter the Sirius at mag 1.4<br /> <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>

 

[search]

Two factors determine the brightness of a star: <br /><br />Luminosity - how much energy it puts out in a given time <br /><br />Distance - how far it is from us <br /><br />A searchlight puts out more light than a penlight. That is, the searchlight is more luminous. If that searchlight is 5 miles away from you, however, it will not be as bright because light intensity decreases with distance squared. A searchlight 5 miles from you may look as bright as a penlight 6 inches away from you.The same is true for stars. <br /><br />Stefan-Boltzmann Law<br />This is the relationship between luminosity (L), radius (R) and temperature (T): <br /><br /><br />L = (7.125 x 10-7) R2 T4<br />Units: L - watts, R - meters, T - degrees Kelvin <br /> <br /><br />Astronomers (professional or amateur) can measure a star's brightness (the amount of light it puts out) by using a photometer or charge-coupled device (CCD) on the end of a telescope. If they know the star's brightness and the distance to the star, they can calculate the star's luminosity [luminosity = brightness x 12.57 x (distance)2]. <br /><br />Luminosity is also related to a star's size. The larger a star is, the more energy it puts out and the more luminous it is. You can see this on the charcoal grill, too. Three glowing red charcoal briquettes put out more energy than one glowing red charcoal briquette at the same temperature. Likewise, if two stars are the same temperature but different sizes, then the large star will be more luminous than the small one.<br />

 

[buckeye101]

Got it! Thanks!

 

[doubletruncation]

I think you're probably mistaking Jupiter for Spica. Jupiter is actually not that far away from Spica right now, and the "speed on to Spica" might point you toward Jupiter just as easily as Spica right now. Jupiter would be quite a bit brighter than any of the other stars in the sky. Spica is slightly to the west of Jupiter. You can find it on this star-chart for the month of August for a location at 40degree north latitude:<br /><br />http://www.skymaps.com/downloads.html<br /><br />Regarding the apparent brightness (or apparent magnitude) charts ranking the brightnesses of different stars - those will be a rough guide to how bright different things might appear to you, but in detail they might not agree with what you perceive with your eyes. The reason is that those charts typically rank the stars by *total* (or bolometric) apparent brightness, which is the total flux in all wavelengths of light. Your eyes are only sensitive to a narrow band of wavelengths (e.g. you can't see infrared or ultraviolet light), so a very hot star that emits most of its light in the ultraviolet might not appear to be as bright to our eyes as a cooler star that emits most of its light in the visible range, even if the total brightness of the hotter star is greater. A classic example is Rigel and Betelgeuse. Rigel in Orion (not Rigel Kentaurus which is Alpha Cen) is actually brighter than Betelgeuse when you consider the entire spectrum of light. However, Rigel is a lot hotter than Betelgeuse and emits much of its light in the UV where you can't see it - to my eyes at least Betelgeuse looks brighter than Rigel. And, there is some subjectivity in determining brightness by eye of stars with different colors. Arcturus is yellower than Vega, so perhaps the whiter star looks brighter to the eye than the yellower one even though we receive more watts per square meter from the yellower one. (A similar thing could happen with <div class="Discussion_UserSignature"> </div>

 

[buckeye101]

broken link trouble, do you have another. Wow i never knew Jupiter was out this month. does anyone have a link to show when the planets are visible and when they're out? That would be cool. I bet it is jupiter, because this thing shines like no other, if it was a planet it should be moving on out of here soon, and that will be evidence it was a planet. I live in Cleveland Ohio so maybe im seeing something different than some of you??? maybe?

 

[buckeye101]

I DOWNLOADED IT TROUBLE I GOT THE SKY MAP--THANKS! How long will Jupiter be around, do you know?

 

[doubletruncation]

Sorry about the link - I guess you can't link directly to the sky-chart pdf, you can find them for each month though on www.skymaps.com. Just look for any of the planets on the chart to see which ones are currently up. You can always use software such as Xephem (or TheSky - which is user friendly but you have to pay for it) to generate a skychart for any location that you please at any time that you please.<br /><br />Regarding how long Jupiter will still be up at night. Note that Jupiter moves pretty slowly with respect to the stars, it takes it about 12 years to orbit the sun. So a good rule of thumb is that it takes ~13 months between times when it transits at midnight (as opposed to 12 months = 1 year for stars), so about half the year it will be up in the evening. Just like the stars it would move ~30 degrees per month (that is it would appear approximately 30 degrees farther towards the west after 1 month when observed at the same time at night), that corresponds to setting about 2 hours earlier after 1 month. So, if you figure out approximately what time it sets now, then you can figure out when it will be setting before sunset (if you don't want to wait up you can approximate by counting the distance in degrees that it has to travel until it sets using the rule that your fist held out at arms length is about 10 degrees). <div class="Discussion_UserSignature"> </div>