M31764

许多业余天文学家认为M3是仅次于M13，在北天最易于观赏的球状星团。M3的视星等是6.2等，使它即使在足够黑暗的环境下，也很难以肉眼看见。使用中等大小的望远镜，可以明确的辨识这个星团。可以使用大角星（牧夫座α）与常陈一（猎犬座α）连线来寻找它，M3几乎就在连线中点的西北方。使用口径 25 cm（9.8英寸）的望远镜，可以看见星团的明亮核心，直径约为6弧分，但它的总跨度是这个的两倍。

这个星团大约由50万颗恒星组成，是已知最大和最亮的星团之一，估计它的年龄约114亿年。它与地球的距离大约是33,900光年。

M 3相当的孤立，它在银河平面上方约31.6 kly（9.7 kpc），距离银河中心约38.8 kly（11.9 kpc）。已知它有274颗变星，这是迄今为止所知拥有最多变星的球状星团。变星中有133颗是天琴座RR型变星，其中大约三分之一显示长周期模式的伯拉兹科效应。除了氢和氦之外，其它元素（天文学家都称之为金属）的总丰度在-1.34〜-1.50dex之间。这个数值提供了相对于太阳的金属量丰度的对数值，实际的比例为太阳丰度的3.2%〜4.6%。M3是奥斯特霍夫I型的原型，这种类型被认为是"金属丰富的"。即对球状星团而言，M3具有相对较高的重元素丰度。

Messier 3 (M3; also NGC 5272) is a globular cluster located 33.9 thousand light years from Earth in the northern constellation of Canes Venatici. It is one of the largest and brightest globular clusters discovered with around 500,000 stars.

Discovery

It was discovered on May 3, 1764, and was the first Messier object to be discovered by Charles Messier himself. Messier originally mistook the object for a nebula without stars. This mistake was corrected after the stars were resolved by William Herschel around 1784. Since then, it has become one of the best-studied globular clusters. Identification of the cluster's unusually large variable star population was begun in 1913 by American astronomer Solon Irving Bailey and new variable members continue to be identified up through 2025.

Visibility

Many amateur astronomers consider it one of the finest northern globular clusters, following only Messier 13. M3 has an apparent magnitude of 6.2, making it a difficult naked eye target even with dark conditions with averted vision. However, with a moderate-sized telescope, the cluster can be seen as a cloudy smudge even in severely light-polluted skies, and can be further defined in darker conditions. It can be found by looking almost exactly halfway along the north-west line that would join Arcturus (α Boötis) to Cor Caroli (α Canum Venaticorum). Using a telescope with a 25 cm (9.8 in) aperture, the cluster has a bright core with a diameter of about 6 arcminutes and spans a total of double that.

Characteristics

This cluster is one of the largest and brightest, and is made up of around 500,000 stars. It is estimated to be 11.4 billion years old. It is centered at 32,600 light-years (10.0 kpc) away from Earth.

Messier 3 is quite isolated as it is 31.6 kly (9.7 kpc) above the Galactic plane and roughly 38.8 kly (11.9 kpc) from the center of the Milky Way. It contains 274 known variable stars, by far the most found in any globular cluster. These include 133 RR Lyrae variables, of which about a third display the Blazhko effect of long-period modulation. The overall abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is in the range of −1.34 to −1.50 dex. This value gives the logarithm of the abundance relative to the Sun; the actual proportion is 3.2–4.6% of the solar abundance. Messier 3 is the prototype for the Oosterhoff type I cluster, which is considered "metal-rich". That is, for a globular cluster, Messier 3 has a relatively high abundance of heavier elements.