حركة خاصة

الحركة الخاصة Peculiar motion هى الحركة الحقيقية لنجم سماوي ما دون أى تأثير لحركة الشمس .

تعد الحركة الخاصة للنجم هى حركته ذاتها خلال الفضاء بالنسبة الى النجوم الآخرى المجاورة .

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 علم الكون

في علم الكون الفيزيائي، السرعة الخاصة تشير إلى مكونات سرعة المجرة التي تشذ عن سريان هبل. وحسب قانون هبل، فالمجرات تنحسر عنا بسرعات تتناسب مع بعدها عنا.

Galaxies are not distributed evenly throughout observable space, but are typically found in groups or clusters, where they have a significant gravitational effect on each other. Velocity dispersions of galaxies arising from this gravitational attraction are usually in the hundreds of kilometers per second, but they can rise to over 1000 km/s in rich clusters. This velocity can alter the recessional velocity that would be expected from the Hubble flow and affect the observed redshift of objects via the relativistic Doppler effect. The Doppler redshift due to peculiar velocities is

$1+z_{pec}={\sqrt {\frac {1+v/c}{1-v/c}}}$

التي هي بالتقريب:

$z\approx v/c$

for low velocities (small redshifts). This combines with the redshift from the Hubble flow to give the observed redshift

$1+z_{obs}=(1+z_{pec})(1+z_{H}).$

The radial velocity of a cosmologically "close" object can be approximated by

$v_{r}=H_{0}d+v_{pec}$

with contributions from both the Hubble flow and peculiar velocity terms, where $H_{0}$ is the Hubble constant and $d$ is the distance to the object.

Redshift-space distortions can cause the spatial distributions of cosmological objects to appear elongated or flattened out, depending on the cause of the peculiar velocities. Elongation, sometimes referred to as the "Fingers of God" effect, is caused by random thermal motion of objects; however, correlated peculiar velocities from gravitational infall are the cause of a flattening effect. The main consequence is that, in determining the distance of a single galaxy, a possible error must be assumed. This error becomes smaller as distance increases. For example, in surveys of type Ia supernovae, peculiar velocities have a significant influence on measurements out to redshifts around 0.5, leading to errors of several percent when calculating cosmological parameters.

Peculiar velocities can also contain useful information about the universe. The connection between correlated peculiar velocities and mass distribution has been suggested as a tool for determining constraints for cosmological parameters using peculiar velocity surveys.

 انظر أيضا

• حركة حقيقية
• سرعة شعاعية Radial velocity
• سرعة نسبية
• Space velocity (astronomy)

 المصادر

• مؤمن, عبد الأمير (2006). قاموس دار العلم الفلكي. بيروت، لبنان: دار العلم للملايين. {{cite book}}: Cite has empty unknown parameter: |طبعة أولى coauthors= (help)