आभासी दृश्यप्रत

आभासी दृश्यप्रत (इंग्रजी: Apparent magnitude; चिन्ह m) ही खगोलीय वस्तूची पृथ्वीवरील निरीक्षकाला भासणारी प्रखरता/तेजस्विता दर्शवणारी संख्या आहे. ही संख्या जितकी लहान तितका तारा जास्त तेजस्वी दिसतो. -२७ आभासी दृश्यप्रत असलेला सूर्य आकाशातील सर्वात तेजस्वी वस्तू आहे. एखादी वस्तू जेवढी जास्त तेजस्वी असते, तिची दृश्यप्रत तेवढी कमी असते. त्याचबरोबर दृश्यप्रत मापणीची पद्धत लॉगॅरिदमिक असते: म्हणजे दृश्यप्रतीमधील एका एककाचा फरक म्हणजे प्रखरतेमध्ये ${\sqrt[{5}]{100}}$ इतक्या म्हणजे २.५१२ पटीचा फरक असतो.

सामान्यत: आभासी दृश्यप्रत सांगण्यासाठी दृश्य वर्णपटाचा आधार घेतला जातो. पण काही वेळा इतर वर्णपटही वापरले जातात (उदा. अवरक्त). दृश्य वर्णपटामध्ये सूर्यानंतर व्याध दुसरा सर्वात तेजस्वी तारा आहे. निकट अवरक्त मधील जे-बॅंडमध्ये काक्षी (भरत) तारा सर्वात तेजस्वी तारा आहे. ताऱ्यांची आभासी दृश्यप्रत बोलोमीटर या यंत्राने मोजतात.

पुढील तक्त्यामध्ये रात्रीच्या आकाशातील किती आभासी दृश्यप्रतीचे तारे उघड्या डोळ्यांनी दिसू शकतात किंवा नाहीत व त्यांची संख्या दिली आहे.

मानवी डोळ्यांना दिसते	आभासी दृश्यप्रत	अभिजित ताऱ्याच्या तुलनेत तेजस्विता	रात्रीच्या आकाशात या आभासी दृश्यप्रतीपेक्षा तेजस्वी दिसणाऱ्या ताऱ्यांची संख्या
होय	−१.०	२५०%	१
	०.०	१००%	४
	१.०	४०%	१५
	२.०	१६%	४८
	३.०	६.३%	१७१
	४.०	२.५%	५१३
	५.०	१.०%	१ ६०२
	६.०	०.४०%	४ ८००
	६.५	०.२५%	९ ०९६
नाही	७.०	०.१६%	१४ ०००
	८.०	०.०६३%	४२ ०००
	९.०	०.०२५%	१२१ ०००
	१०.०	०.०१०%	३४० ०००

मोजमाप

पृथ्वीच्या वातावरणाचा आभासी तेजस्वितेवर परिणाम होतो. त्यामुळे आभासी दृश्यप्रत मोजताना एखादी खगोलीय वस्तू विना वातावरणाची जेवढी तेजस्वी दिसली असती ती किंमत देतात. प्रखरता अंतरानुसार बदलते. त्यामुळे अतिशय तेजस्वी वस्तू ती खूप लांब असेल तर कमी तेजस्वी दिसू शकते. एखाद्या खगोलीय वस्तूची निरपेक्ष दृश्यप्रत म्हणजे ती वस्तू १० पार्सेक किंवा ३२.६ प्रकाशवर्ष अंतरावर आहे असे गृहीत धरले असतानाची तिची आभासी दृश्यप्रत होय.

x बॅंडमधील आभासी दृश्यप्रत m पुढील समीकरणाने देतात:

$m_{x}-m_{x,0}=-2.5\log _{10}\left({\frac {F_{x}}{F_{x,0}}}\right)\,$,

जिथे $F_{x}\!\,$ हा x बॅंडमधील अभिवाह (फ्लक्स) आहे आणि $m_{x,0}$ व $F_{x,0}$ हे त्याच बॅंडमधील संदर्भासाठीचे आभासी दृश्यप्रत आणि अभिवाह आहेत, जसे की अभिजित ताऱ्याचे.

उदा: सूर्य आणि चंद्र

सूर्य आणि चंद्राच्या आभासी दृश्यप्रतींचे गुणोत्तर काय आहे?

सूर्याची आभासी दृश्यप्रत -२६.७४ (तेजस्वी) आहे, तर पौर्णिमेच्या चंद्राची सरासरी आभासी दृश्यप्रत -१२.७४ (कमी तेजस्वी) आहे.

दृश्यप्रतेमधील फरकः

$x=m_{1}-m_{2}=(-12.74)-(-26.74)=14.00$

प्रखरतेतील तफावत:

$v_{b}=10^{0.4x}=10^{0.4*14.00}\approx 398400$

सूर्य पौर्णिमेच्या चंद्रापेक्षा ~४०,००० पट जास्त तेजस्वी दिसतो.

काही खगोलीय वस्तू

माहीत असलेल्या खगोलीय वस्तूंच्या आभासी दृश्यप्रती

आभासी दृश्यप्रत (V)	खगोलीय वस्तू
−40.98	Rho Cassiopeiae as seen from 1 astronomical unit (AU).
−38.00	Rigel as seen from 1 AU. It would be seen as a large very bright bluish scorching ball of 35° apparent diameter.
−30.30	Sirius as seen from 1 astronomical unit
−29.30	Sun as seen from Mercury at perihelion
−27.40	Sun as seen from Venus at perihelion
−26.74	Sun as seen from Earth (about 400,000 times brighter than mean full moon)
−25.60	Sun as seen from Mars at aphelion
−23.00	Sun as seen from Jupiter at aphelion
−21.70	Sun as seen from Saturn at aphelion
−20.20	Sun as seen from Uranus at aphelion
−19.30	Sun as seen from Neptune
−18.20	Sun as seen from Pluto at aphelion
−16.70	Sun as seen from एरिस (बटु ग्रह) at aphelion
−14.2	An illumination level of one lux
−12.90	Maximum brightness of perigee+perihelion full moon (mean distance value is −12.74, though both values are about 0.18 magnitude brighter when including the opposition effect)
−11.20	Sun as seen from Sedna at aphelion
−10	Comet Ikeya–Seki (1965), which was the brightest Kreutz Sungrazer of modern times
−9.50	Maximum brightness of an Iridium (satellite) flare
−7.50	The SN 1006 supernova of AD 1006, the brightest stellar event in recorded history (7200 light years away)
−6.50	The total integrated magnitude of the night sky as seen from Earth
−6.00	The Crab Supernova (SN 1054) of AD 1054 (6500 light years away)
−5.9	International Space Station (when the ISS is at its perigee and fully lit by the Sun)
−4.89	Maximum brightness of Venus when illuminated as a crescent
−4.00	Faintest objects observable during the day with naked eye when Sun is high
−3.99	Maximum brightness of Epsilon Canis Majoris 4.7 million years ago, the historical brightest star of the last and next five million years
−3.82	Minimum brightness of Venus when it is on the far side of the Sun
−2.95	Maximum brightness of Mars
−2.70	Maximum brightness of Jupiter
−2.50	Faintest objects visible during the day with naked eye when Sun is less than 10° above the horizon
−2.50	Minimum brightness of new moon
−2.45	Maximum brightness of Mercury at superior conjunction (unlike Venus, Mercury is at its brightest when on the far side of the Sun, the reason being their different phase curves)
−1.61	Minimum brightness of Jupiter
−1.47	Brightest star (except for the Sun) at visible wavelengths: Sirius
−0.83	Eta Carinae apparent brightness as a supernova impostor in April 1843
−0.72	Second-brightest star: Canopus
−0.49	Maximum brightness of Saturn at opposition and perihelion when the rings are full open (2003)
−0.27	The total magnitude for the Alpha Centauri AB star system. (Third-brightest star to the naked eye)
−0.04	Fourth-brightest star to the naked eye Arcturus
−0.01	Fourth-brightest individual star visible telescopically in the sky Alpha Centauri A
+0.03	Vega, which was originally chosen as a definition of the zero point
+0.50	Sun as seen from Alpha Centauri
1.47	Minimum brightness of Saturn
1.84	Minimum brightness of Mars
3.03	The SN 1987A supernova in the Large Magellanic Cloud 160,000 light-years away.
3 to 4	Faintest stars visible in an urban neighborhood with naked eye
3.44	The well known Andromeda Galaxy (M31)
4.38	Maximum brightness of Ganymede (moon of Jupiter and the largest moon in the Solar System)
4.50	M41, an open cluster that may have been seen by Aristotle
5.20	Maximum brightness of asteroid Vesta
5.32	Maximum brightness of Uranus
5.72	The spiral galaxy M33, which is used as a test for naked eye seeing under dark skies
5.73	Minimum brightness of Mercury
5.8	Peak visual magnitude of gamma-ray burst GRB 080319B (the "Clarke Event") seen on Earth on March 19, 2008 from a distance of 7.5 billion light-years.
5.95	Minimum brightness of Uranus
6.49	Maximum brightness of asteroid Pallas
6.50	Approximate limit of stars observed by a mean naked eye observer under very good conditions. There are about 9,500 stars visible to mag 6.5.
6.64	Maximum brightness of dwarf planet Ceres in the asteroid belt
6.75	Maximum brightness of asteroid Iris
6.90	The spiral galaxy M81 is an extreme naked-eye target that pushes human eyesight and the Bortle scale to the limit
7 to 8	Extreme naked-eye limit, Class 1 on Bortle scale, the darkest skies available on Earth
7.78	Maximum brightness of Neptune
8.02	Minimum brightness of Neptune
8.10	Maximum brightness of Titan (largest moon of Saturn), mean opposition magnitude 8.4
8.94	Maximum brightness of asteroid 10 Hygiea
9.50	Faintest objects visible using common 7x50 binoculars under typical conditions
10.20	Maximum brightness of Iapetus (brightest when west of Saturn and takes 40 days to switch sides)
12.91	Brightest quasar 3C 273 (luminosity distance of 2.4 billion light years)
13.42	Maximum brightness of Triton
13.65	Maximum brightness of Pluto (725 times fainter than magnitude 6.5 naked eye skies)
15.40	Maximum brightness of centaur Chiron
15.55	Maximum brightness of Charon (the large moon of Pluto)
16.80	Current opposition brightness of Makemake
17.27	Current opposition brightness of Haumea
18.70	Current opposition brightness of एरिस
20.70	Callirrhoe (small ~8 km satellite of Jupiter)
22.00	Approximate limiting magnitude of a 24" Ritchey-Chrétien telescope with 30 minutes of stacked images (6 subframes at 300s each) using a CCD detector
22.91	Maximum brightness of Pluto's moon Hydra
23.38	Maximum brightness of Pluto's moon Nix
24.80	Amateur picture with greatest magnitude: quasar CFHQS J1641 +3755 and limit of a 30-second exposure with Large Synoptic Survey Telescope (LSST).
25.00	Fenrir (small ~4 km satellite of Saturn)
27.70	Faintest objects observable in a 10-hour image with a 8-meter class ground-based telescope such as the Subaru Telescope
28.00	Jupiter if it were located 5000 au from the Sun
28.20	Halley's Comet in 2003 when it was 28 au from the Sun
31.50	Faintest objects observable in visible light with Hubble Space Telescope
35.00	LBV 1806-20, a luminous blue variable star, expected magnitude at visible wavelengths due to interstellar extinction
36.00	Faintest objects observable in visible light with E-ELT
हेही बघा: List of brightest stars