玉衡 (恒星)
 在大熊座的玉衡(Alioth)。 | |
| 觀測資料 曆元 J2000 | |
|---|---|
| 星座 | 大熊座 |
| 星官 | 紫微垣 北斗 |
| 赤經 | 12h 54m 01.6s |
| 赤緯 | +55° 57′ 35.4″ |
| 視星等（V） | 1.76 |
| 特性 | |
| 光谱分类 | A0pCr |
| U−B 色指数 | 0.02 |
| B−V 色指数 | -0.02 |
| 变星类型 | alpha2-CVn |
| 天体测定 | |
| 徑向速度 (Rv) | -9.3 km/s |
| 自行 (μ) | 赤经：111.74 mas/yr 赤纬：-8.99 mas/yr |
| 视差 (π) | 40.3 ± 0.62 mas |
| 距离 | 81 ± 1 ly (24.8 ± 0.4 pc) |
| 绝对星等 (MV) | -0.22 |
| 詳細資料 | |
| 質量 | ~3 M☉ |
| 半徑 | 3.7 R☉ |
| 亮度 | 108 L☉ |
| 溫度 | 9,400 K |
| 金屬量 | ? |
| 自轉 | 38 km/s. |
| 年齡 | ? 年 |
| 其他命名 | |
玉衡星（ε UMa / 大熊座ε）是大熊座最亮的恒星，也是北斗七星之一，會隨著獵犬座α型的變星的影響每隔5.089日而變光為1.76等～1.78等亮。它距離地球約80光年。It is the star in the tail of the bear closest to its body, and thus the star in the handle of the Big Dipper closest to the bowl. It is also a member of the large and diffuse Ursa Major moving group. Historically, the star was frequently used in celestial navigation in the maritime trade, because it is listed as one of the 57 navigational stars.
语源和文化象征
英文名Alioth，源于阿拉伯语alyat—，原意是“熊的尾巴”。在中文古代天文中，它被称作北斗五（星官北斗的第五星）；在《史記》的天官書中稱為玉衡，而唐代的密教教典《佛說北斗七星延命經》中則稱為廉貞，原文如下：“南無廉貞星。是東方淨住世界廣達智辨如來佛。”
特性
According to Hipparcos, Alioth is 81 light years (25 parsecs) from Earth. Its spectral type is A0p; the "p" stands for peculiar, as the spectrum of its light is characteristic of an Alpha2 Canum Venaticorum variable. Alioth, as a representative of this type, may harbor two interacting processes. First, the star's strong magnetic field separating different elements in the star's hydrogen 'fuel'. In addition, a rotation axis at an angle to the magnetic axis may be spinning different bands of magnetically-sorted elements into the line of sight between Alioth and the Earth. The intervening elements react differently at different frequencies of light as they whip in and out of view, causing Alioth to have very strange spectral lines that fluctuate over a period of 5.1 days.
With Alioth, the rotational and magnetic axes are at almost 90 degrees to one another. A map of the star below shows how darker (denser) regions of chromium form a band at right angles to the equator.
A recent study suggests Alioth's 5.1-day variation may be due to a substellar object of about 14.7 Jupiter masses in an eccentric orbit (e=0.5) with an average separation of 0.055 astronomical units.
Alioth has a relatively weak magnetic field, fifteen times weaker than α CVn, but it is still 100 times stronger than that of the Earth.
参见
参考资料
- This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.
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