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Brightness vs luminosity<\/h4>\n\n
All brightness is relative. Scott Levine <\/strong>reveals how we categorise our favourite stars <\/p>\n\n One of the first things many of us learn when we start stargazing is that Sirius (Alpha Canis Majoris) <\/span>is night\u2019s brightest star. But what does it mean to say that something is bright, and what\u2019s the difference between brightness and luminosity? <\/p>\n\n Astronomers use different terms to talk about how much light we see from celestial objects and how much light those objects actually produce. But first, let\u2019s talk about magnitude. <\/p>\n\n We generally talk about celestial objects in terms of magnitude<\/strong>. This doesn\u2019t measure how much light exists; rather it describes objects\u2019 brightness relative to each other. The Greek astronomer Hipparchus devised a scale in the second century BC, grouping stars by their brightness: the brightest stars at 1 and the dimmest stars at 6.<\/span><\/p>\n\n Today, objects with lower magnitudes are still listed as brighter than objects with higher magnitudes, and we use negative numbers for the brightest objects. So a star of mag. \u20131 is brighter than a star of mag. +2. Objects of about magnitude +1.5 or brighter are \u2018first-magnitude\u2019, objects from +1.5 to +2.5 are second, +2.5 to +3.5 are third, and so on. <\/p>\n\n Apparent magnitude,<\/strong> visual magnitude or simply magnitude, is the brightness we see when we observe our favourite stars. Hipparchus set Vega (Alpha Lyrae) at 0.0 on this scale and all other magnitudes are generally based on it (Vega\u2019s has since been refined to +0.03). Under dark skies, stars as dim as +6.5 are visible with the naked eye. The Sun\u2019s apparent magnitude is \u201326.74. It\u2019s the brightest object in our sky by far. The average full Moon\u2019s is about \u201312.75, and at its brightest Venus\u2019s apparent magnitude reaches about \u20134.9. <\/p>\n\n But just as a neighbour\u2019s window lamp appears brighter than a distant streetlight, what we see is a combination of how much light an object creates, and its distance. So two identical stars at different distances can have different apparent magnitudes.<\/span><\/p>\n\n While apparent magnitude gives us an idea of how we see stars, no matter their distance, absolute magnitude<\/strong> puts them on equal footing. Instead of saying, \u201cI think that one\u2019s brighter,\u201d we compare how bright objects would be if they were all 10 parsecs (32.6 lightyears) away (about the same distance as Arcturus (Alpha Bo\u00f6tis). <\/span><\/p>\n\n We can then talk about objects in terms of how much light they produce: how luminous <\/strong>they are. Some very distant stars create so much energy that we see them as among the night\u2019s brightest, despite them being so far away.<\/span><\/p>\n\n With this in mind, let\u2019s go back to the Sun. It\u2019s bright enough to bring us daylight, but its absolute <\/strong>magnitude is only about +4.83. That\u2019s about the same as the apparent <\/strong>magnitudes of Delta Ursae Minoris at +4.34, and Eta Ursae Minoris at +4.95. Both are more luminous than the Sun. From Earth, though, they\u2019re quite faint, just like most of Ursa Minor\u2019s stars, and they give us an idea of what our Sun would look like if it lay near Arcturus. <\/p>\n\n Two of the sky\u2019s brightest stars, Sirius and Rigel (Beta Orionis), appear near each other and are a spectacular sight on a dark night. But Sirius (apparent <\/strong>magnitude \u20131.46) is about 8.6 lightyears away, while Rigel (apparent<\/strong> magnitude +0.13) is far behind, around 700 lightyears. <\/p>\n\n Sirius is about twice the size of the Sun and has an absolute <\/strong>magnitude of +1.43. Meanwhile, Rigel a supergiant 18\u201324 times more massive than the Sun, has an absolute<\/strong> magnitude of \u20137.84. So while nearby Sirius appears brighter, the absolute magnitudes tell us distant Rigel is more luminous, and far outshines the brightest star in the night sky. <\/p>\n\n Next time you look up at the stars, consider absolute and apparent magnitude and imagine just how bright the stars you\u2019re observing actually are. <\/p>\n\n The sky\u2019s brightest stars are not always its most luminous. Here are the five brightest stars visible from the UK, sorted by their apparent magnitude (brightness<\/strong>) and absolute magnitude (luminosity<\/strong>) <\/p>\n\n\n\n Scott Levine <\/strong>is an astronomy writer and naked-eye observer based in New York\u2019s Hudson Valley.<\/p>\n\n <\/p>\n\n All brightness is relative. We explain what that means when it comes to categorising our favourite stars<\/p>\n","protected":false},"author":24,"featured_media":37487,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"ub_ctt_via":"","purple_page_number":"72","purple_custom_meta_purple_page_number":"72","purple_seq_number":"1","purple_custom_meta_purple_seq_number":"1","purple_source_article":"article_72-1.xml","purple_custom_meta_purple_source_article":"article_72-1.xml","purple_source_issue":"November-2022","purple_custom_meta_purple_source_issue":"November-2022","purple_external_id":"November-2022-72-1","purple_custom_meta_purple_external_id":"November-2022-72-1","purple_issue_code":"|0000086558||","purple_custom_meta_purple_issue_code":"|0000086558||","purple_android_product":"com.im.skyatnight.210","purple_custom_meta_purple_android_product":"com.im.skyatnight.210","purple_ios_product":"com.im.skyatnight.210","purple_custom_meta_purple_ios_product":"com.im.skyatnight.210","purple_web_product":"","purple_custom_meta_purple_web_product":"","purple_publication_id":"075fab74-0a21-4201-866a-899d6c41c40c","purple_migrated":"","kt_blocks_editor_width":""},"categories":[21],"tags":[14],"featured_image_src":"https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3.jpg","author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"acf":{"readingTimeMinutes":"5","apple_news_title":""},"uagb_featured_image_src":{"full":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3.jpg",1039,907,false],"thumbnail":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3-150x150.jpg",150,150,true],"medium":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3-300x262.jpg",300,262,true],"medium_large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3-768x670.jpg",768,670,true],"large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3-1024x894.jpg",800,698,true],"1536x1536":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3.jpg",1039,907,false],"2048x2048":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/10\/2-3.jpg",1039,907,false]},"uagb_author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"uagb_comment_info":0,"uagb_excerpt":"All brightness is relative. We explain what that means when it comes to categorising our favourite stars","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/37251"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"replies":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/comments?post=37251"}],"version-history":[{"count":11,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/37251\/revisions"}],"predecessor-version":[{"id":37950,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/37251\/revisions\/37950"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media\/37487"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=37251"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=37251"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/tags?post=37251"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/dj9jqhxgw9833.cloudfront.net\/uploads\/sites\/77\/2022\/10\/Layer-21.png\")
What is magnitude? <\/strong><\/h5>\n\n
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Distance and luminosity <\/strong><\/h5>\n\n
How bright are the brightest stars?<\/h4>\n\n\n\n
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