Luminosità solare

Evoluzione della luminosità solare, del raggio e della temperatura effettiva rapportata alla situazione al 2010. Ribas (2010)[1]

In astronomia, la luminosità solare standard è un'unità di misura, usata per esprimere la luminosità delle stelle e di oggetti più grandi quali le nebulose ad emissione.

Descrizione

La luminosità solare è un'unità di misura del flusso radiante emesso sotto forma di fotoni e utilizzata in astronomia per misurare la luminosità di stelle, galassie e altri oggetti celesti confrontandola con l'emissione dal Sole.

La luminosità nominale è definita dall'Unione Astronomica Internazionale e il suo valore e il suo simbolo convenzionale sono:[2]

Questo valore non include il contributo del neutrino solare che aggiungerebbe 8,8×1024 W,[3] portando la luminosità totale a 3,916×1026 W.

Il Sole è classificabile come una stella debolmente variabile, per cui la sua luminosità tende a fluttuare.[4] Le fluttuazioni solari seguono un ciclo di variazione periodica quasi undecennale causate dalla presenza delle macchie solari che comporta una fluttuazione di circa ±0.1%. Ci sono anche altre variazioni collegate a cicli di 200-300 anni, ma con valori molto inferiori.[5]

Il Sole risulta in effetti dall'analisi statistica una stella di grandezza e luminosità intermedie nella popolazione stellare. Per questo motivo, i suoi parametri descrittivi (massa e luminosità in particolare) sono utilizzati come unità di misura per oggetti astronomici di dimensioni comparabili, come le altre stelle, o maggiori come le galassie.

Conoscendo la densità e la massa di una galassia, e presupponendo che sia costituita in prevalenza da stelle come il Sole, si può dedurre il numero di stelle che la compongono.

Note

  1. ^ Ignasi Ribas, The Sun and stars as the primary energy input in planetary atmospheres (PDF), in Solar and Stellar Variability: Impact on Earth and Planets, Proceedings of the International Astronomical Union, IAU Symposium, vol. 264, febbraio 2010, pp. 3–18, Bibcode:2010IAUS..264....3R, DOI:10.1017/S1743921309992298, arXiv:0911.4872.
  2. ^ Resolution B3 on recommended nominal conversion constants for selected solar and planetary properties (PDF), su iau.org, International Astronomical Union, 2015. URL consultato il 5 giugno 2018.
  3. ^ John N. Bahcall, Neutrino Astrophysics, Cambridge University Press, 1989, p. 79, ISBN 978-0-521-37975-5.
  4. ^ L. E. A. Vieira, A. Norton, T. Dudok De Wit, M. Kretzschmar, G. A. Schmidt e M. C. M. Cheung, How the inclination of Earth's orbit affects incoming solar irradiance (PDF), in Geophysical Research Letters, vol. 39, n. 16, 2012, p. L16104 (8 pp.), Bibcode:2012GeoRL..3916104V, DOI:10.1029/2012GL052950, insu-01179873.
  5. ^ Peter D. Noerdlinger, Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System, in Celestial Mechanics and Dynamical Astronomy, vol. 801, 2008, p. 3807, Bibcode:2008arXiv0801.3807N, arXiv:0801.3807.

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