{"id":603,"date":"2022-03-02T17:45:23","date_gmt":"2022-03-02T17:45:23","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/astronomy\/?post_type=chapter&p=603"},"modified":"2022-03-02T17:45:24","modified_gmt":"2022-03-02T17:45:24","slug":"summary-16","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/astronomy\/chapter\/summary-16\/","title":{"raw":"Summary","rendered":"Summary"},"content":{"raw":"
\r\n

19.1<\/span>\u00a0Fundamental Units of Distance<\/span><\/h3>\r\n

Early measurements of length were based on human dimensions, but today, we use worldwide standards that specify lengths in units such as the meter. Distances within the solar system are now determined by timing how long it takes radar signals to travel from Earth to the surface of a planet or other body and then return.<\/p>\r\n\r\n<\/section>

\r\n

19.2<\/span>\u00a0Surveying the Stars<\/span><\/h3>\r\n

For stars that are relatively nearby, we can \u201ctriangulate\u201d the distances from a baseline created by Earth\u2019s annual motion around the Sun. Half the shift in a nearby star\u2019s position relative to very distant background stars, as viewed from opposite sides of Earth\u2019s orbit, is called the parallax of that star and is a measure of its distance. The units used to measure stellar distance are the light-year, the distance light travels in 1 year, and the parsec (pc), the distance of a star with a parallax of 1 arcsecond (1 parsec = 3.26 light-years). The closest star, a red dwarf, is over 1 parsec away. The first successful measurements of stellar parallaxes were reported in 1838. Parallax measurements are a fundamental link in the chain of cosmic distances. The Hipparcos satellite has allowed us to measure accurate parallaxes for stars out to about 300 light-years, and the Gaia mission will result in parallaxes out to 30,000 light-years.<\/p>\r\n\r\n<\/section>

\r\n

19.3<\/span>\u00a0Variable Stars: One Key to Cosmic Distances<\/span><\/h3>\r\n

Cepheids and RR Lyrae stars are two types of pulsating variable stars. Light curves of these stars show that their luminosities vary with a regularly repeating period. RR Lyrae stars can be used as standard bulbs, and cepheid variables obey a period-luminosity relation, so measuring their periods can tell us their luminosities. Then, we can calculate their distances by comparing their luminosities with their apparent brightnesses, and this can allow us to measure distances to these stars out to over 60 million light-years.<\/p>\r\n\r\n<\/section>

\r\n

19.4<\/span>\u00a0The H\u2013R Diagram and Cosmic Distances<\/span><\/h3>\r\n

Stars with identical temperatures but different pressures (and diameters) have somewhat different spectra. Spectral classification can therefore be used to estimate the luminosity class of a star as well as its temperature. As a result, a spectrum can allow us to pinpoint where the star is located on an H\u2013R diagram and establish its luminosity. This, with the star\u2019s apparent brightness, again yields its distance. The various distance methods can be used to check one against another and thus make a kind of distance ladder which allows us to find even larger distances.<\/p>\r\n\r\n<\/section>\r\n

This book was adapted from the following: Fraknoi, A., Morrison, D., & Wolff, S. C. (2016). Summary In Astronomy<\/i>. OpenStax. https:\/\/openstax.org\/books\/astronomy\/pages\/19-summary under a Creative Commons Attribution License 4.0<\/a><\/div>\r\n
Access the entire book for free at\u00a0https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction<\/a><\/div>","rendered":"
\n

19.1<\/span>\u00a0Fundamental Units of Distance<\/span><\/h3>\n

Early measurements of length were based on human dimensions, but today, we use worldwide standards that specify lengths in units such as the meter. Distances within the solar system are now determined by timing how long it takes radar signals to travel from Earth to the surface of a planet or other body and then return.<\/p>\n<\/section>\n

\n

19.2<\/span>\u00a0Surveying the Stars<\/span><\/h3>\n

For stars that are relatively nearby, we can \u201ctriangulate\u201d the distances from a baseline created by Earth\u2019s annual motion around the Sun. Half the shift in a nearby star\u2019s position relative to very distant background stars, as viewed from opposite sides of Earth\u2019s orbit, is called the parallax of that star and is a measure of its distance. The units used to measure stellar distance are the light-year, the distance light travels in 1 year, and the parsec (pc), the distance of a star with a parallax of 1 arcsecond (1 parsec = 3.26 light-years). The closest star, a red dwarf, is over 1 parsec away. The first successful measurements of stellar parallaxes were reported in 1838. Parallax measurements are a fundamental link in the chain of cosmic distances. The Hipparcos satellite has allowed us to measure accurate parallaxes for stars out to about 300 light-years, and the Gaia mission will result in parallaxes out to 30,000 light-years.<\/p>\n<\/section>\n

\n

19.3<\/span>\u00a0Variable Stars: One Key to Cosmic Distances<\/span><\/h3>\n

Cepheids and RR Lyrae stars are two types of pulsating variable stars. Light curves of these stars show that their luminosities vary with a regularly repeating period. RR Lyrae stars can be used as standard bulbs, and cepheid variables obey a period-luminosity relation, so measuring their periods can tell us their luminosities. Then, we can calculate their distances by comparing their luminosities with their apparent brightnesses, and this can allow us to measure distances to these stars out to over 60 million light-years.<\/p>\n<\/section>\n

\n

19.4<\/span>\u00a0The H\u2013R Diagram and Cosmic Distances<\/span><\/h3>\n

Stars with identical temperatures but different pressures (and diameters) have somewhat different spectra. Spectral classification can therefore be used to estimate the luminosity class of a star as well as its temperature. As a result, a spectrum can allow us to pinpoint where the star is located on an H\u2013R diagram and establish its luminosity. This, with the star\u2019s apparent brightness, again yields its distance. The various distance methods can be used to check one against another and thus make a kind of distance ladder which allows us to find even larger distances.<\/p>\n<\/section>\n

This book was adapted from the following: Fraknoi, A., Morrison, D., & Wolff, S. C. (2016). Summary In Astronomy<\/i>. OpenStax. https:\/\/openstax.org\/books\/astronomy\/pages\/19-summary under a Creative Commons Attribution License 4.0<\/a><\/div>\n
Access the entire book for free at\u00a0https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction<\/a><\/div>\n","protected":false},"author":33,"menu_order":11,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-603","chapter","type-chapter","status-publish","hentry","chapter-type-numberless"],"part":591,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/603","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/users\/33"}],"version-history":[{"count":1,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/603\/revisions"}],"predecessor-version":[{"id":604,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/603\/revisions\/604"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/parts\/591"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/603\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/media?parent=603"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapter-type?post=603"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/contributor?post=603"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/license?post=603"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}