Messier 28 information

Messier 28
Messier 28
	Globular cluster Messier 28 in Sagittarius
Observation data (J2000 epoch)
Class	IV
Constellation	Sagittarius
Right ascension	18h 24m 32.89s
Declination	–24° 52′ 11.4″
Distance	18.26 ± 0.98 kly (5.6 ± 0.3 kpc)
Apparent magnitude (V)	6.8
Apparent dimensions (V)	11′.2
Physical characteristics
Mass	5.51×105 M☉
Radius	30 ly
VHB	15.55 ± 0.10
Metallicity	= –1.32 dex
Estimated age	12.0 Gyr
Notable features	Contains first pulsar discovered in a globular
Other designations	GCl 94, M 28, NGC 6626
	See also: Globular cluster, List of globular clusters

Messier 28 or M28, also known as NGC 6626, is a globular cluster of stars in the center-west of Sagittarius. It was discovered by French astronomer Charles Messier in 1764.^[11]^[a] He briefly described it as a "nebula containing no star... round, seen with difficulty in 31⁄2-foot telescope; Diam 2′."^[12]

In the sky it is less than a degree to the northwest of the 3rd magnitude star Kaus Borealis (Lambda Sgr). This cluster is faintly visible as a hazy patch with a pair of binoculars^[11] and can be readily found in a small telescope with an 8 cm (3.1 in) aperture, showing as a nebulous feature spanning 11.2 arcminutes. Using an aperture of 15 cm (5.9 in), the core becomes visible and a few distinct stars can be resolved, along the periphery. Larger telescopes will provide greater resolution,^[5] one of 25 cm (9.8 in) revealing a dense 2′ core, with more density within.^[11]

It is about 18,300 light-years away from Earth.^[3] It is about 551000 M_☉^[6] and its metallicity (averaging −1.32 which means more than 10 times less than our own star), coherency and preponderence of older stellar evolution objects, support its dating to very roughly 12 billion years old.^[9] 18 RR Lyrae type variable stars have been found within.

It bore the first discovery of a millisecond pulsar in a globular cluster – PSR B1821–24. This was using the Lovell Telescope at Jodrell Bank Observatory, England.^[10] A total of 11 further of these have since been detected in it with the telescope at Green Bank Observatory, West Virginia. As of 2011, these number the third-most in a cluster tied to the Milky Way, following Terzan 5 and 47 Tucanae.^[13]

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^ ^a ^b ^c "M 28". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.
^ ^a ^b Cite error: The named reference Oliveira was invoked but never defined (see the help page).
^ "Messier 28". SEDS Messier Catalog. Retrieved 28 April 2022.
^ ^a ^b Cite error: The named reference inglis2004 was invoked but never defined (see the help page).
^ ^a ^b ^c Cite error: The named reference apj742_1_51 was invoked but never defined (see the help page).
^ From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 30 ly
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^ ^a ^b Cite error: The named reference mnras385_4_1998 was invoked but never defined (see the help page).
^ ^a ^b "JBO - Stars". Jodrell Bank Observatory. Retrieved 2007-06-01.
^ ^a ^b ^c Cite error: The named reference thompson2007 was invoked but never defined (see the help page).
^ Cite error: The named reference burnham1979 was invoked but never defined (see the help page).
^ Cite error: The named reference apj730_2_81 was invoked but never defined (see the help page).

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[hcob849_11-1] Cite error: The named reference hcob849_11 was invoked but never defined (see the help page).

[simbad-2] "M 28". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.

[Oliveira-3] Cite error: The named reference Oliveira was invoked but never defined (see the help page).

[4] "Messier 28". SEDS Messier Catalog. Retrieved 28 April 2022.

[inglis2004-5] Cite error: The named reference inglis2004 was invoked but never defined (see the help page).

[apj742_1_51-6] Cite error: The named reference apj742_1_51 was invoked but never defined (see the help page).

[7] From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 30 ly

[aj121_2_916-8] Cite error: The named reference aj121_2_916 was invoked but never defined (see the help page).

[mnras385_4_1998-9] Cite error: The named reference mnras385_4_1998 was invoked but never defined (see the help page).

[jbo_stars-10] "JBO - Stars". Jodrell Bank Observatory. Retrieved 2007-06-01.

[thompson2007-11] Cite error: The named reference thompson2007 was invoked but never defined (see the help page).

[burnham1979-13] Cite error: The named reference burnham1979 was invoked but never defined (see the help page).

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